Chapter 27

Fig. 390.—Tongue, Proboscis, and piercing apparatus of Drone-fly (Eristalis tenax).

Fig. 391.—Under-surface of a Wasp’s Tongue, Feelers, &c. (Seen within the circle is the tongue about life-size.)

In the drone-fly (Eristalis tenax), the mouth organs are larger than in the house-fly, and differ in many respects. The tongue is split up for a certain distance, and then again united, as represented inFig. 390. The labium, mandibles, and maxillæ are converted into well-developed lancet-shaped organs; these both pierce the skin of animals, and form tubes by which their blood may be sucked up. Next to the maxillary palpi a couple of lancets are seen to project out; these again are associated with two other instruments, one resembling in appearance a two-edged sword, and a peculiar one with pincers or cutting teeth at the extremity. It is very peculiar, and resembles an instrument used in surgery for enlarging the wound, and in this case to increase the flow of blood. This remarkable compound piercing apparatus of the drone-fly is of exquisite finish, and must strike the observer with amazement, while it greatly transcends the work of human mechanism. The fleshy tongue itself projects somedistance from the apparatus described, and is furnished with setæ or hairs, shorter and fewer in number than those of the house-fly, and while its spiral structure is not so fully developed, its retractor, muscles, and ligaments are even more so.

The further development of the mouth organs must be looked for in other members of the insect tribe, when it will be seen many assume a more or less modified form of structure, that, for example, in Hymenoptera (the bee and wasp), in which insects the mouth and tongue are divided into lobes which are used to extract the nectary (as Linnæus termed it) from the plants on which they feed. The tongue in most species is capable of extension and contraction.

Fig. 392.1. Sting of Wasp (Vespa vulgaris), with its muscular attachments and palpi for cleansing the apparatus; 2. Sting of Bee.

Fig. 392.

1. Sting of Wasp (Vespa vulgaris), with its muscular attachments and palpi for cleansing the apparatus; 2. Sting of Bee.

InFig. 391the under-surface of the wasp’s tongue is shown, together with its two pairs of antennæ, and pair of brushes on either side, for brushing off the gathered pollen and honey from the broad tongue. It is amply provided with muscular structure. The antennæ, or feelers, are as curious in form as they are delicate in structure. Those of the male differ from those of the female.

Both the bee and the wasp are armed with an exceedingly venomous sting, as is well known. This structure takes the form of a well-adapted mechanical contrivance, and is a weapon of offence as well as of defence. The sting consists of two barbed needle-points, of a sufficient length to pierce the flesh to some depth. From the peculiar arrangement of their serrated edges their immediate withdrawal cannot take place, and it is this circumstance, with the drop of poison injected into the open wound, that renders their sting of the most painful and irritating kind. The gland containing the poison is contained in a minute sac situated at the root of the piercing apparatus. InFig. 392is shown the sting of the wasp and the bee.

Very many insects are provided with instruments for boring into the bark or solid wood itself. The female Cynip bores into the oak-apple for the purpose of depositing her egg. The larva, when full grown, eats its way out of the nut, and drops to the ground, where it attains the form of the perfect fly (Fig. 393).

Fig. 393.—Female Gall-fly and Larva.

There are numbers of species living exclusively upon the leaves of plants, to which they do much damage by the excrescences or galls they form. Each tree seems to be infested by its own species of gall-mite, the so-callednail-gallof the lime being caused by a species namedPhytoptus tibiæ. These galls take the form of a pointed column, standing erect on the upper side of the leaf. Galls of much the same structure occur in the sycamore, maple, elm, and various fruit trees.

The gnat (Culex pipiens) is furnished with a sting curiously constructed (Fig. 394), and enclosed in a perfectly clothed sheath covered throughout by scales or feathers. This is folded up when not in use. The mouth is provided with a complete set of lancets for piercing the flesh; after having inflicted a severe wound, it injects an acid poison through the proboscis. The scales of the gnat vary in structure accordingly as these are found on the wing, the body, or the proboscis. A magnified wing is shown at No. 2,Fig. 394, and a magnified scale from the proboscis at No. 3. InFig. 405, Nos. 3 and 5, more highly magnified wing and body scales are given. The proboscis is protected on either side by antennæ and feelers.

Fig. 394.1. Head ofCulex pipiens, female Gnat, detached from body; 2. Wing, showing nervature and fringed edges; 3. Scale from Proboscis; 4. Proboscis and Lancets. The reticulated markings on each side of the head show the proportionate space occupied by the eyes.

Fig. 394.

1. Head ofCulex pipiens, female Gnat, detached from body; 2. Wing, showing nervature and fringed edges; 3. Scale from Proboscis; 4. Proboscis and Lancets. The reticulated markings on each side of the head show the proportionate space occupied by the eyes.

The giant-tailed wasp,Sirax gigas, is furnished with an even more curious mechanical boring apparatus (Fig. 395) than its congeners. This is a boring ovipositor, skilfully contrived for piercing the bark of trees, in which the insect deposits her eggs, and where the larva, when hatched, will find an ample supply of food to carry it through this stage of existence. The boring tube, it will be seen, is a perfect muscular structure (c,c,a, andx); in short, it is an endless form of drill, well known to the mechanic, such as is employed in fine work for drilling holes. The females are of some size, and maybe surprised and taken in the act of boring through the bark of the pine tree, for which they have a preference.

Fig. 395.—Boring apparatus of Giant-tailed Wasp (Sirex gigas), × 350.

There is also a species of the broad-bodied saw-fly,Lyda campestris. These bore the Scotch fir, and deposit their eggs. The larvæ from these eggs, when hatched out, feed upon the pine-needles, first spinning a fine web to conceal their work of depredation. A better known saw-fly,Abraxas grossulariata, plays havoc among our gooseberry trees. The female is provided with a curious mechanical apparatus as an ovipositor, with which she cuts into the thicker under-leaf of the plant. This penetrating and cutting tool consists of a double-saw (Fig. 396) of elaborate construction, which when not in use is kept concealed in a long narrow case situated beneath the abdomen. It is further protected by two horny plates. The saws pass out through a deep groove so arranged that the saws work side by side backwards and forwards, without a possibility of running out of the groove. When the cut is made, the four are drawn together and form a central canal, through which an egg is forced into the leaf. The cutting edges of the saws are provided with about eighteen or twenty teeth; these have sharp points of extreme delicacy, and together make a serrated edge of the exact form given to the finest and best-made surgical saws of the present day. In the summer-time the proceedings of the female insect may be witnessed, and the method of using this curious instrument seen, by the aid of a hand magnifier. These insects are not easily alarmed when busy at work.

Fig. 396.—Saws of the Gooseberry-fly (Abraxas grossulariata).

Before bringing my remarks on proboscides of insects to a conclusion, attention must be given to that of the honey bee (Apis mellifica), and its curious accessories. The mouth of bees exhibits a combination of the suctorial and the masticatory form of oralapparatus. Thus the labial, or upper lip, and the mandibles, or large pair of jaws, are well developed, while the maxillæ, or lesser pair, are elongated to form a tubular organ, through which, together with the tongue, the flower juices, “honey-dew,” may be sucked up. The labium, lower lip, is also rather prolonged, and the palpi, or organs of touch, with which it is endowed form a useful protective apparatus. The mandibles are employed by bees in the construction of their abodes, while the suctorial portion of the mouth is devoted to the reception of nourishment and to prehension. The sting of the bee, already noticed, is in fact an ovipositor, the female alone being provided with this weapon as an egg-depositing organ, although better known as anaculeusor sting; but it forms no part of the oral apparatus (as shown inFig. 397). The proboscis itself will be seen to be curiously divided; the divisions are elegant and regular, beset with numerous setæ or hairs. The two horny outside lancets are spear-shaped and partially set with short hairs; at the base of each is a hinge articulation; this permits of considerable motion in several directions, and is much used by the busy insect for forcing open the more internal parts of flowers, thus facilitating theintroduction of the proboscis. The two shorter feelers are closely connected with the proboscis, and terminate in three-jointed articulations. The structure of the proboscis is so arranged that it can be enlarged at the base, and thus made to contain a greater quantity of the collected honey-dew; at the same time it is in this cavity the nectar appears to be converted into pure honey. The proboscis tapers off to a little nipple-like extremity, and at its base is seen two shorter and stronger mandibles, from between which is protruded a long andnarrow lance-like tongue, the whole being most curiously connected by a series of strong muscles and ligaments. The basal or first joint of the hind leg in the neuter or working bee is developed into an enlarged form of pocket, used by the insect for conveying the pollen of flowers and the propolis to the hive. Indeed, both the tibia and the first joint of the tarsus are broadened out into plates, but the two sides of the plates are differently furnished. On one side is a thick coating of hairs, those on the tarsus taking the form of a brush, evidently used for brushing out the pollen, as these special developments are not found on the hind legs of the drones or of the queen.

Fig. 397.1. Honey bee’s tongue; 2. Leg of worker bee. (The small circles show the objects about the natural size.)

Fig. 397.

1. Honey bee’s tongue; 2. Leg of worker bee. (The small circles show the objects about the natural size.)

Fig. 398.1. Foot and leg of Ophion; 2. Foot and leg of Flesh-fly; 3. Foot and leg of Drone-fly, with pad or sucker appendage.

Fig. 398.

1. Foot and leg of Ophion; 2. Foot and leg of Flesh-fly; 3. Foot and leg of Drone-fly, with pad or sucker appendage.

The wax used in the formation of cells is a secretion that exudes through certain portions of the body of the bee, since it is found in little pouches situated on the under part of the body, but it is not brought home ready for use. The walls of the cells are strengthened when completed by a kind of varnish, already referred to as the propolis, collected from the buds of poplar and lime trees, and this is spread over the walls of the cell by that wonderful pair of broad spatulæ, represented in the drawing.

Many interesting variations will be found in the legs and feet of flies, as well as in those of other orders of insects (Lepidoptera). One or two typical forms are represented inPlate VI., and inFig. 398.

Fig. 399.—Sucker on the leg of Water-beetle. (The dot in the circle represents the object natural size.)

The tarsus, or foot of the fly (Fig. 398), consists of a deeply bifid, membranous structure,pulvillus; anterior to its attachment to the fifth tarsal joint, or the upper surface, are seated two claws, or “tarsal ungues”; these are freely movable in every direction. These ungues differ greatly in their outline, size, and relative development to the tarsi, and to the bodies of the insects possessing them, and in their covering; most are naked over their entire surface, having however a hexagonal network at their bases, which indicates a rudimentary condition of minute scale-like hairs, such as are common on some part of the integument of all insects. Flexor and extensor muscles are attached to both ungues and flaps; the flaps are either corrugated or arranged on the ridge and furrow plan, in other cases they are perfectly smooth on their free surface, while others are covered with minute scale-like hairs. The thickness of the divided membrane on the blow-fly does not exceed the1⁄2000th of an inch at the margin; they somewhat increase in thickness towards the point of attachment. Projecting from the flap are organs which have been termed “hairs,” “hair-like appendages,” “trumpet-shaped hairs.” These are doubtless the immediate agents in holding on to a smooth surface, as that of glass, and are termed “tenent-hairs,” in allusion to their office. The under surface of left forefoot ofMusca vomitoriais shown with tenent-hairs (Plate VI., No. 140);aandbare more magnified hairs,afrom below,bfrom the side. No. 142 is the left forefoot ofAmara communis, showing the under surface and form of tenent appendages, one of which is seen more magnified ata; No. 143, under surface of left forefoot,Ephydra riparia. This fly is met with in immense numbers on the surface water in salt marshes. It does not possessthe power of climbing glass; this is explained by the structure of the tenent-hairs; the central tactile organ is also very peculiar, the whole acting as a float, one to each foot, to enable the fly to rest on the surface of the water;ais one of the external hairs, No. 135, under surface of left forefoot ofCassida viridis(tortoise-beetle), showing the bifurcate tenent appendages, one of which is given atamore magnified. These, in ground beetles, are met with only in males, and are used for sexual purposes. The delicacy of the structure of these hairs in the fly and the elastic membranous expansion of the foot are marvellous. When the fly is climbing, a minute quantity of some glutinous fluid is exuded, so that the tubular nature of the tenent-hairs hardly admits of a doubt.

“At the root of the pulvillus, or its under surface, is a process, which in some instances is short and thick, in others long and curved, and tapering to its extremity (Scatophaga), setose (Empis), plumose (Hippoboscidæ), or, in one remarkable example (Ephydra), closely resembling in its appearance the very rudimentary pulvillus with which it is associated. Just at the base of the fifth tarsal joint, on its under surface, there is present, in Eristalis, a pair of short, very slightly curved hairs, which point almost directly downwards.”82

Tenent-hairs are usually present in some modification or other. It is really difficult to name a beetle which has not some form of them; the only one I yet know that seems to me really to possess nothing of the kind is a species of Helops, living on sandy heaths. I suppose the dense cushion of hairs on the tarsi to be for the protection, simply, of the joints to which they are attached. I have detected them on the tarsal joints of species of Ephydra, and on the first basal tarsal joint of the drone of the hive-bee. A very rudimentary form of tenent-hairs is present on the under surface of some of the tree-bugs (Pentatomidæ), which have in addition a large, deeply-cleft organ at the extremity of the tarsus; this appears to be a true sucker.

When walking on a rough surface, the foot represents that of a Coleopterous insect without any tenent appendages. The ungues are always attached to the last joint of an insect’s tarsus. They are not attached to the fifth tarsal joint of a Dipterous insect, neither are they attached to the fifth tarsal joint of a Hymenopterousinsect, but to the terminal sucker, which again, in this great order, is a sixth tarsal joint, membranous, flexible, elastic in the highest degree, retractile to almost its fullest extent within the fifth tarsal joint—a joint modified to an extraordinary degree for special purposes.

In plantula of Lucanus, with its pair of minute claws, the ungues are hairs modified for special purposes; and they have the structure of true hairs. The sustentacula of Epeira, the analogous structures on the entire under surface of the last tarsal joints in Pholcus, the condition of the parts in the hind limbs of Notonecta, in both its mature and earlier conditions, as well as in Sarcoptes, Psoroptes, and some other Acari, all may be cited in proof of this fact. The various orders of insects have, for the most part, each their own type of foot. Thus there is the Coleopterous type, the Hymenopterous type, the Dipterous type, the Homopterous type, &c.; each so very distinctive, that in critical instances they will sometimes serve at once to show to which order an insect should be referred. Thus, amongst all the Diptera, I have as yet met with but one subdivision which presents an exception to the structure described. This exception is furnished by the Tipulidæ, which have the Hymenopterous foot. With hardly an exception, then, I believe the form of foot described will be found universal among the Diptera.

It may be desirable to add a few words on the best plan of conducting observations on the feet of insects. Their action should be studied by placing the insect under the influence of chloroform. It is of advantage to carefully preserve the parts examined, and for this purpose Deane’s medium or glycerine jelly suits very well; some of the more delicate preparations, however, can only be kept unchanged in a solution of chloride of zinc. The plan of soaking in caustic potash, crushing, washing, putting into spirits of wine and then into turpentine, and lastly into Canada balsam, is perfectly useless, excepting in rare instances where points connected with the structure of the integument have to be made out. Of course, the parts should be viewed from above, from below, and in profile, in order to gain exact ideas of their relations. The binocular microscope diminishes the difficulties which formerly had to be encountered, as by its aid many parts may be clearly viewed without preparation of any kind.

Fig. 400.1. Antenna of the Silkworm-moth; 2. Tongue of Butterfly; 3. A portion of tongue highly magnified, showing its muscular fibre; 4. Tracheæ of silkworm; 5. Foot of silkworm. (The small circles enclose each object somewhat near the natural size.)

Fig. 400.

1. Antenna of the Silkworm-moth; 2. Tongue of Butterfly; 3. A portion of tongue highly magnified, showing its muscular fibre; 4. Tracheæ of silkworm; 5. Foot of silkworm. (The small circles enclose each object somewhat near the natural size.)

Moths and butterflies supply the microscopist with some of the most beautiful objects for examination. What can be more wonderful in its adaptation than the antenna of the moth (represented inFig. 400, No. 1), with a thin, finger-like extremity almost supplyingthe insect with a perfect and useful hand, moved throughout its extent by a muscular apparatus of the most exquisite construction. The tongue of butterfly (No. 2) is evidently made for the purpose of dipping into the interior of flowers and extracting the juices; this act is assisted by a series of fine muscles. An enlarged view of a portion is given at No. 3; seePlate VI., Nos. 132 and 133, antennæ of Vapour Moth.

Fig. 401.—Breathing aperture or spiracle of silkworm. (In the circle it is shown about the natural size.)Fig. 402.—Magnified portions of the trachea of the Hydrophilus, showing spiral tubes.

Fig. 401.—Breathing aperture or spiracle of silkworm. (In the circle it is shown about the natural size.)

Fig. 402.—Magnified portions of the trachea of the Hydrophilus, showing spiral tubes.

The inconceivably delicate structure of the maxillæ or tongues (for there are two) of the butterfly, rolled up like the trunk of an elephant, and capable, like it, of every variety of movement, has been carefully examined and described by Mr. Newport. “Each maxilla is convex on its outer surface, but concave on its inner; so that when the two are united they form a tube,haustellium, by their union, through which fluids may be drawn into the mouth. The inner or concave surface, which forms the tube, is lined with a very smooth membrane, and extends throughout the whole length of the organ; while that of each maxilla is hollow in its interior, apparently forming a tube ‘in itself,’ but this is not so; the mistake has arisen from the existence of large tracheæ, or breathing tubes, in the interior of the proboscis. In some species the extremity of the haustellium is studded externally with a number of minute papillæ, or fringes—as inVanessa atalanta—in which they becomesmall elongated barrel-shaped bodies, terminated by smaller papillæ at their extremities. On alighting on a flower, the insect makes a powerful expiratory effort, by which the air is expelled from the interior air-tubes, and from those with which they are connected in the head and body; and at the moment of applying its proboscis to the food, it makes an inspiratory effort, by which the central canal in the proboscis is dilated, and the food ascends it at the same instant to supply the vacuum produced; and thus it passes into the mouth and stomach, the constant ascent of the fluid being assisted by the action of the muscles of the proboscis, which continues during the whole time that the insect is feeding. By this combined agency of the acts of respiration and the muscles of the proboscis we are also enabled to understand the manner in which the humming-bird sphynx extracts in an instant the honey from a flower while hovering over it, without alighting; and which it certainly would be unable to do were the ascent of the fluid entirely dependent upon the action of the muscles of the organ.”

The trachæal or respiratory system of insects varies, or rather is found to exist in modified forms to suit their varied conditions of life. While in the larval stage the breathing apertures are seen to recur at intervals on each side of the abdomen (as that of the silkworm,Fig. 401), thus ensuring a continuous supply of air to the circulating fluids throughout the whole body. These spiracles are usually nine or ten in number, and consist of a membranous ring of an oval form. The air-tubes are exquisitely composed of two thin membranes, between which a delicate elastic thread or spiral fibre is interposed, forming a cylindrical opening and keeping the tube in a distended condition, thus mechanically preserving the sides from collapse or pressure in their passage through the air, which otherwise might occasion suffocation.Fig. 402represents the double spiral arrangement of a portion of a trachea of Hydrophilus, which ensures both elasticity and strength.

There are other points of interest confined to the water-beetle tribe, among the more striking of which is the foreleg of theDytiscus marginalis. Here the first three joints of the tarsus are expanded into a broad surface, and fringed throughout with curved hairs. From the surface of these spring a number of short hairs, with cup-like discs at their extremities, one of which is seen highly magnified inPlate VI.,No. 142. These are so cup-like in form that they have been hitherto described as “suckers,” but it is believed they are simply a special apparatus for the development of the hairs seen on the leg and foot of the beetle. Another curious example occurs in the Gyrinus, or whirligig-beetle. The front pair of legs are of the ordinary kind, but the under pair are furnished with expanding paddles. The trochanter, femur, and tibia, are flat plates of a triangular shape, pointed at their outer angles, from which the apex springs. But the tarsus is jointed on the inner angle of the furthermost end of the tibia, and each of its four joints expands into a flat paddle blade. In the accompanyingFig. 403one paddle is seen expanded, the other closed.

Fig. 403.1. Leg of Gyrinus, Whirligig, paddle shown expanded. 2. Paddle closed up.

Fig. 403.

1. Leg of Gyrinus, Whirligig, paddle shown expanded. 2. Paddle closed up.

These paddles are adapted with much precision to ensure the most effectual application of the propelling power; as the beetle strikes out in the act of swimming, the membranous expansion described enables it to move about with great rapidity; upon the legs being drawn back towards the body, the membrane closes up, and thus offers no resistance to the water. The eyes are not the least curious part of the merry little beetle: the upper section is fitted for seeing in the air, and is adapted to the upper or superiorpart of the head; the lower portion, for seeing under the water, being placed at a lower angle, a thin division only separating the two.

Fig. 404.—Scales from Butterflies’ and Moths’ wings, magnified 200 diameters.1. Scale ofMorpho menelaus; 2. Large scale ofPolyommatus argiolus, azure blue; 3.Hipparchia janira argiolus; 4.Pontia brassica; 5.Podura plumbea; 6. Small scale of azure blue.

Fig. 404.—Scales from Butterflies’ and Moths’ wings, magnified 200 diameters.

1. Scale ofMorpho menelaus; 2. Large scale ofPolyommatus argiolus, azure blue; 3.Hipparchia janira argiolus; 4.Pontia brassica; 5.Podura plumbea; 6. Small scale of azure blue.

Wings of Insects.—These exhibit variety of form and structure, as well as of beauty of colouring. At an early period the orders of insects were mainly founded upon these interesting appendages. The Orthoptera were the straight wings; the Neuroptera the nerved; the Trichoptera the hairy wings; the Coleoptera the cased or sheathed wings; the Diptera the two wings; the Hymenoptera the married wings; and the Lepidoptera the scaled wings. A number of wings are small and membranous, and may be mounted dry for examination under the microscope. Others are better seen mounted in benzol-balsam. The elytra, iridescent wing cases of the diamond, and other beetles, as well as the wings of the more highly coloured butterflies, make pretty objects mounted dry for opaqueillumination by the Lieberkühn or reflector. The thicker horny cases of other members of the beetle tribe require long soaking, as described in a former chapter.

The wings of moths and butterflies are covered with scales or feathers, carefully overlapping each other, as tiles are made to cover the tops of houses. The iridescent variety of colouring on insects’ wings arises from the peculiar wavy arrangement of the scales. Figs. 404 and 405 are magnified representations of a few of them. No. 1, a scale of theMorpho menelaus, taken from the side of the wing, is of a pale-blue colour; it measures about1⁄120th of an inch in length, and exhibits a series of longitudinal striæ or lines, between which are disposed cross-lines or other striæ, giving it very much an appearance of brick-work (better seen inFig. 405, No. 1).

Fig. 405.—Portions of Scales, magnified 500 diameters.1. Portion of scale ofMorpho menelaus;2. Portion of large scale ofPodura plumbea;3. Scale from the wing of Gnat, its two layers being represented; 4. Portion of a large scale ofLepisma Saccharina;5. Body scale of Gnat, magnified 650 diameters.

Fig. 405.—Portions of Scales, magnified 500 diameters.

1. Portion of scale ofMorpho menelaus;2. Portion of large scale ofPodura plumbea;3. Scale from the wing of Gnat, its two layers being represented; 4. Portion of a large scale ofLepisma Saccharina;5. Body scale of Gnat, magnified 650 diameters.

Polyommatus argiolus, azure-blue (Fig. 404, Nos. 2 and 6), are large and small scales taken from the under-side of the wing of this beautiful blue butterfly; the small scale is covered with a series of spots, and exhibits both longitudinal and transverse striæ, these should be clearly defined, and the spots separated by a quarter-inch object-glass. No. 3,Hipparchia janira, is a scale from the meadow-brown butterfly:on this brown spots, having an irregular shape with longitudinal striæ, are seen. No. 4,Pontia brassica, cabbage butterfly, was at one time taken to be an excellent criterion of the penetration and definition of an object glass. It is seen to have a free extremity or brush-like appendage. With a fairly good power, the longitudinal markings appear like rows of small beads. Chevalier selected for his test object the scale of thePontia brassica. Mohl and Schacht extolledHipparchia janiraas a good test of penetration in an objective of moderate angular aperture. Amici’s test object isNavicula rhomboides, the display of the lines forming the test.

Fig. 406.—Podura villosa, male and female, highly magnified.

TheTinea vestianella, clothes-moth, is furnished with unique scales. Small and destructive as this moth is, it suffers much from a parasitic mite, and from which it is unable to free itself.

The Podura scale (Fig. 405), with its delicate transparent membrane and curiously inserted “notes of admiration,” as they were called, was long believed to be an excellent test object for the highest powers of the microscope, but I believe it is no longer regarded in that light: indeed, most insect scales have declined in the value and estimation of the skilled microscopist. This is in part due to the improvements made in the objective. The high-angled glasses have cleared up obscure points in the structural characters of the minuter forms of life, and the scales of insects are no longer found to be difficult test objects for the modern objective of a Zeiss or a Powell to resolve. Nevertheless, the scale of the Podura belonging to the order Thysanura, a curious little insect commonly known by the name of springtail, usually found living in most obscure places, and too small to attract attention, is not likely to be entirely thrust aside. The springtails (Collembola) are furnished on the under-side of the first abdominal segment with a curious tube or sucker, from theorifice of which glandular process a secreted viscid matter is protruded; they are remarkable also from the fact that in most of them no trace of a tracheal system has yet been discovered. The eyes when present are in the form of simple or grouped ocelli, the antennæ number six joints, and the abdomen has but six segments, often only three. The forked tail is a curious process turned forward and attached to one of the tender segments and held in position under the body; when released it springs back and bounds up to a very considerable height.Fig. 406representsPodura villosa. There are several species, one of which (P. aquatica) is found floating in patches on pools of water on bright summer days.

Lepisma saccharinabelongs to the same genus as Podura. This minute springtail derives its name from having been discovered in old sugar-casks. It has a spindle-shaped body covered with silvery scales, long used as test objects. The sides of the abdomen are furnished with a series of appendages with long bristle-like setæ, or hairs, at their extremities. The head is concealed under a prothorax, the antennæ are long, and the maxillary palpi are either five or seven-jointed, and very conspicuous, to enable them to cut the dry wood on which they principally feed. The scales must be mounted under thin cover-glasses; oblique illumination shows up some portions to advantage, while central light from an achromatic condenser and a wide-angled objective renders their markings more distinct. Portion of a scale more highly magnified is shown inFig. 405.

Eggs of Insects(Plate VI., Nos. 124-139).—In form, colour, and variety of design, the eggs of insects are more surprisingly varied than those of the feathered tribes; but as from extreme smallness they escape observation, an acquaintance with their structure is not so familiar as it might be. Although the eggs of the bird tribe differ much in their external characteristics, they closely resemble each other while yet a part of the ovarian ova, and prior to their detachment from the ovary. At one period of their formation all eggs consist of three similar parts:—1st. The internal nucleated cell, or germinal vesicle, with its macula; 2nd. The vitellus, or yolk-substance; and 3rd. The vesicular envelope, or vitelline membrane. The germinal vesicle is the first produced, then the yolk substance, which gradually envelops it, and the vitelline membrane, the latestformed, incloses the whole. The chemical constituents of the egg are the same in all cases, albumen, fatty matters, and a proportion of a substance precipitable by water. The production of the chorion, or shell membrane, does not take place till the ovum has attained nearly its full size, and it then appears to proceed, in part at least, from the consolidation over the whole surface of one or more layers of an albuminous fluid secreted from the wall of the oviduct.

The embryo cell is so directly connected with the germinal vesicle that at a certain period it disappears altogether, and is absorbed into the germinal yolk, or rather becomes the nucleus of the embryo, when a greater degree of compactness is observed in the yolk, and all that remains of the germinal vesicle is one or more highly refracting fat globules and albuminoid bodies. Towards the end of the period of incubation, the head of the young caterpillar is said to lie towards the dot or opening in the lid, termed the micropyle,83from its resemblance to a small gate, or opening through which the larva emerges forth as a butterfly.

The germinal vesicle is comparatively large and well-marked while the egg is yet in the ova-sac. By preparing sections after Dr. Halifax’s method,84we find that the germinal vesicle in the bee’s egg is not situated immediately near or even below the so-called micropyle, but rather more to the side of the egg; just in the position which the head of the embryo is subsequently found to occupy at maturity.

The egg membrane, or envelope, of all the Lepidoptera is composed of three separate and distinct layers: an external slightly raised coat, tough and hard in its character, a middle one of united cells, and a fine transparent vitelline lining membrane, perfectly smooth and homogeneous in structure, imparting solidity, and giving a fine iridescent hue to the surface. The germinal vesicle is of aproportionately large size for the egg, and its macula is at first single, then multiple. In the egg of the silkworm the outer membrane is comprised of an inner reticulated membrane of non-nucleated cells, in the outer layer the cells are arranged in an irregular circular form, also non-nucleated, with minute interstitial setæ or hairs projecting outward.

The outer surface of the egg-shell ofCoccus Persicæis covered by minute rings, of which the ends somewhat overlap. These rings are thought to be identical in their character with the whitish substance which exudes through pores on the under-side of the body; it is more than probable that a succession of layers of rings fully accounts for the beautiful prismatic hues they present viewed as opaque objects under the microscope, and illuminated by Lieberkühn or side-condenser. This white substance, it should be observed, forms a part of the intimate structure of the egg-shell, and is in nowise affected by methylated spirit or dilute acids. Sir John Lubbock85states that in the greenish eggs of Phryganea, “the colour is due to the yolk-globules themselves. In Coccus, however, this is not so; the yolk-globules are slightly yellow, and the green hue of the egg is owing to the green granules, which are minute oil globules. When, however, the egg arrives at maturity, and the upper chamber has been removed by absorption, these green granules will be found to be replaced by dark-green globules, regular in size, and about1⁄8000th of an inch in diameter, and which appear to be in no way the same in the yolk of Phryganea eggs.” Another curious fact has been noticed, which partially bears on the question of colour: the production of parasite bodies within the eggs of some insects. In the Coccus, for instance, parasitic cells of a green colour occur, “shaped like a string of sausages, in length about the1⁄2000th of an inch by about the1⁄7000th in breadth.”

The eggs of moths and butterflies present many varying tints of colour; in speaking of this quality I do not restrict the term solely to those prismatic changes to which allusion has been made, and which are liable to constant mutations according to the accident of the rays of light thrown upon them; but I more particularly refer to the several natural transitions of colour, the prevailing tints of which are yellow, white, grey, and a light-brown. In some eggsthe yellow, white, and grey are delicately blended, and, when viewed with a magnifying power of about fifty diameters, and by the aid of the side-reflector (parabolic-reflector), exhibit many beautiful combinations. The more delicate opalescent, or rather iridescent, tints appear on the eggs of insects, while those of the feathered tribes furnish no like example. The egg of the mottled umber moth,Erannis defoliaria(Plate VI., No. 137), is in every way very beautiful. It is in shape ovoid, with regular hexagonal reticulations, each corner being studded with a knob or button; the space within the hexagon is finely punctated, and the play of colours is exquisitely delicate. In this egg no micropyle can be seen. The egg of the thorn moth,Ennomos erosaria(Plate VI., No. 138), is of an elongated brick-looking form, one end of which is slightly tapered off, while the other, in which the lid is placed, is flattened and surrounded by a beautifully white-beaded border, having for its centre a slightly raised reticulated micropyle. The empty egg-shell gives a fine opalescent play of colours, while that containing the young worm is of a brownish-yellow.

The egg of the straw-belle moth,Aspillates gilvaria(Plate VI., No. 139), is delicately tinted, somewhat long and narrow, with sides slightly flattened or rounded off, and irregularly serrated. The top is convex, and the base a little indented, in which are seen the lid and micropyle. The young worm, however, usually makes its way through the upper convex side: the indentation represented in the drawing shows the place of exit.

An example of those eggs possessing a good deal of natural colour is presented in that of the common puss-moth,Cerura vinula, a large spheroidal-shaped egg, having, under the microscope, the appearance of a fine ripe orange; the micropyle exactly corresponds to the depression left in this fruit on the removal of the stalk. The surface is finely reticulated, and the natural colour a deep orange.

The egg of the mottled rustic moth,Caradina morpheus(No. 124), is subconical, and equally divided throughout by a series of ribs, which terminate in a well-marked geometrically-formed lid. The egg of the tortoise-shell butterfly,Vanessa urticæ(No. 125), is ovoid and divided into segments, the ribs turning in towards the micropyle. The common footman,Lithosia campanula(No. 126), produces a perfectly globular egg covered with fine reticulationsof a delicate buff colour. The egg of the shark moth,Cucullia umbratica(No. 127), is subconical in form, with ribs and cross-bars passing up from a flattened base to the summit, and turning over to form the lid. No. 136 is the egg of blue argus butterfly,Polyommatus argus. That of the small emerald moth,Jodis Vernaria(No. 134), is an egg of singular form and beauty—an oval, flattened on both sides, of silvery iridescence, and covered throughout with minute reticulations and dots. It is particularly translucent, so much so that the yellow-brown worm is readily seen curled up within. The lid or micropyle is not detected until the larva eats its way out of the shell. It should be noted that the series of eggs inPlate VII. are somewhat over-coloured, and consequently lose much of their natural transparency. The eggs of flies and parasites also present much variety in form, colour, and construction. Many of their eggs are provided with a veritable lid, which opens up with a hinge-like articulation. This lid is seen in the egg of bot-fly,Plate VI., No. 144, from which the larva is just escaping; No. 146, egg of Scatophaga; No. 147, egg of parasite of magpie.86Still more remarkable in the delicate and beautiful forms are some of the parasities which infest birds in particular:Plate VI., No. 145, the egg of parasite of pheasant; No. 147, that of the magpie, while that of the peacock is curiously interesting. InFig. 407the larvæ of the horn-bill are seen just about to emerge from their eggs.

Fig. 407.—Larvæ of the Hornbill emerging from eggs.

The larvæ of most Hymenoptera are footless grubs, furnished with a soft head, and exhibiting but little, if any, advance upon those of Diptera (Plate VI., No. 141). In the saw-fly, however, the larva, instead of being as above described, a mere footless maggot, presents the closest resemblance to the caterpillar of the Lepidoptera; it is provided with a distinct head, with six thoracic legs, and in mostcases from twelve to sixteen pro-legs are appended to the abdominal segments.

One other conspicuous object represented inPlate VI., No. 128, is the maple Aphis, also known as the leaf-insect, averaging in size about the one-fiftieth of an inch in length. Although recognised and described under the name of the leaf-insect, nothing was known of its origin and history, with the exception of what the Rev. J. Thornton published in 1852, and to whom we owe its re-discovery on the leaves of the maple. Subsequently it attracted the attention of the Dutch naturalist, Van der Hoeven, who regarded it as the larval form of a species of Aphis, and named it Periphyllus. It has more recently engaged the attention of Dr. Balbiani and M. Siguoret, whose united investigations will be found in “Comptes Rendus,” 1867. These observers assigned it definitely to Aphis. A brown species is also met with during a great part of the year feeding upon the young shoots of the maple. The female produces two kinds of young, as do all the genus Aphis, one normal the other abnormal; the first are alone capable of reproducing their species, while the latter retain their original form, which is not changed throughout their existence. They increase so slowly in size that it may appear doubtful whether they eat, the mouth being rudimentary; they undergo no change; do not acquire wings, and their antennæ always retain the five joints peculiar to all young Aphides before the first moult. Neither are they all of the same colour, some being of a bright green, as represented inPlate VI., while others are of a darker, or brownish-green colour. The brown-green embryos differ from the adult female only in those characters analogous to all other species, and this chiefly with regard to the minute hairs, which are long and simple. In the green embryos, in the place of setæ, the body is surrounded by transparent lamellæ, oblong in shape. These scales not only cover the body, but also the anterior portion of the head, the first joint of the antennæ, and the outer edge of the tibiæ of the first pair of legs. The dorsal surface in these insects is covered with a mosaic of hexagonal plates, very closely resembling the plates of the carapace of the tortoise. In this particular my artist has fallen into a slight error. Another peculiarity is that the body is much flattened out, and looks so much like a scale on the surface of the leaf that it requires considerable practice, as well as quickness of sight, to detectthe young maple Aphis. One of the lamellæ is seen highly magnified atc, and a tenent-hair atb. The antennæ, tapering off towards the apex, are serrate on both edges, and terminate in a fine lancet (shown ata), with which it penetrates the leaf of the plant. Beneath the insertions of the antennæ is a complex form of sucking mouth, and on either side of the head are two brilliant scarlet-coloured eyes.

Aphides, as is well known, live upon the juices of plants, which they suck, and when they occur in great numbers cause considerable damage to the gardener and farmer. Many plants are liable to be attacked by swarms of these insects, when their leaves curl up, they grow sickly, and their produce is either greatly reduced or utterly ruined. One striking instance is presented in the devastation caused by the hop-fly (Aphis humuli).

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