THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC.
Fig. 208.—Larva of Dryocampa rubicunda, stage II.—Bridgham del.
The cuticula.—The integument is externally either smooth and shining or variously punctured, granulated, tuberculated, striated, or hairy. In certain orders the skin is clothed with flattened setæ or scales, while many forms, as some caterpillars (Figs. 208, 209), beetles (Fig. 210), etc., are protected by spines, horns, etc., these in adult insects often forming secondary sexual characters, usually being more developed in the males than in the females.
Fig. 209.—Larva of Hyperchiria io, on hatching.
The cuticula is not always smooth, but is often finely granulated or even minutely spinulated. On the abdominal segments of Anabrus, as observed by Minot, the cuticula is armed with microscopic conical nodules scattered irregularly over it. They do not correspond, he says, in any way to hairs; for they do not rest over pores, nor did he see any specially modified cells underlying them. “As far as I have observed, they are mere local irregularities, each nodule being apparently supported by some four or six unmodified epidermal cells.” Minot adds that the whole of the cuticula, except the cones just described and the hairs, is divided into numerous minute fields, each of which corresponds to a single cell of the underlying hypodermis. Each field is bounded by a distinct polygonal outline, and its surface is either covered by a large number of extremely minute projecting points, as on the dorsal arch of the segment, or is smooth, as upon the articular membrane and ventral arch. Upon the sides of the dorsal arch and upon the spiracular membrane each field has a projecting spine or sometimes two or even three. (See also pp. 28, 30.)
Fig. 210.—Phanæus pegasus, ♂, from Mexico.—After Graber.
Fig. 211.—Section of integument of Datana ministra: c, cuticula; hyp, hypodermis; p, outer pigmented nodulated layer.
The cuticle of lepidopterous larvæ has also been described and figured by Minot. In the caterpillars of different groups investigated by him, the cuticle was found to be rough with microscopic teeth or spinules, erect or flattened and scale-like, and either densely crowded or scattered, and affording excellent generic and specific characters. In the slug-worms (Limacodids) we have observed that the cuticula is unusually rough, especially on the spiniferous tubercle of Empretia, Parasa, etc. (Fig. 213, c). The skin of the body between the tubercles is seen to be finely shagreened, due to the presence of fine teeth, which are more or less curved and bent, these teeth arising from a very finely granulated surface (d). The cuticle of neuropterous, trichopterous, and tenthredinid larvæ will probably afford similar cases. The integument of the larva of Datana is, on the black bands, rough and nodulated, the irregular nodules being filled with a black pigment, and forming a layer (p) external to the true cuticula (Fig. 211).
Fig. 212.—Hairs of Datana: f, formative hair-cell; c, cuticula; p, pigmented layer; hy, hypodermis.
The integument of many insects contains fine canals passing through the chitinous layers and opening externally in minute pores. Certain of the pore-canals communicate with hollow setæ which sit directly over the pores; other pores form the external openings of dermal glands, but in many cases they are empty or only filled with air, and do not have any hairs connected with them. Each of these pores communicates with a hair-forming hypodermal cell, called by Graber a trichogen.
Setæ (“hairs” and bristles).—The setæ of insects are, as in worms, processes of the cuticle originating from certain of the hypodermal cells. They arise either from a ring-like pit, or from a minute tubercle, and are usually situated at the outlet of a pore-canal, which connects with an underlying cell of the hypodermis (Fig. 212). They are, then, bristle or hair-like processes arising from the hypodermis. Where the hairs or setæ are rubbed off, their site is indicated by a minute ring like a follicle in the chitinous integument. The cuticular hair, says Leydig, is in its first condition the secretion of the cellular element of the skin, and a thread-like continuation of the cell-body may rise up through the pore-canal into the centre of the hair, remaining there permanently.
While the setæ are usually simple, they are often branched, plumose, or spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ of certain coccinellid beetles, notably Epilachna, and of Cassida, the larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia, Colletes, Apis, etc.).
The use of these spinulose, plumose, and twisted hairs in the bees is clearly shown by J. B. Smith, who states that as these insects walk over flowers, the pollen grains adhere to the vestiture, “and this also accounts for the fact, probably noticed by every observant fruit-grower, that bees frequently bury themselves completely in the blossoms, or roll over every part of them. Such insects are after pollen, not honey, and by so rolling about, the pollen grains are brought into contact with and adhere to the surface of the insect.” The syrphid flies also pollenize flowers, the pollenizing of chrysanthemums being effected, as Smith states, by Eristalis tenax, and he adds that the body vestiture of the syrphids “is often composed of spurred and branched hairs.” (For reference to gathering hairs, see p. 45.)
Fig. 213.—Cuticular spinules of larva of Adoneta: a, b, c, d, different forms; e, e′, caltrops.
Certain remarkable spines occur in limacodid larvæ, notably Empretia and Adoneta. These we have called caltrops spines, from their resemblance to the caltrops formerly used in repelling the attacks of cavalry. They are largely concerned in producing the poisonous and irritating effects resulting from contact with the caterpillars of these moths, and are situated in scattered groups near the end of the tubercles. A group of three is represented at Fig. 213, e. They are not firmly embedded in the cuticle, but on the contrary appear to become very easily loosened and detached, and they probably, when brought into contact with the skin of any aggressor, burrow underneath, and are probably in part the cause of the continual itching and annoyance occasioned by these creatures. It will be seen by reference to Fig. 213, e′, that the body of the spine is spherical, with one large, elongated, conical spine arising from it, the spherical base being beset with a number of minute, somewhat obtuse spinules.
Fig. 214.—Glandular hairs of caterpillars. A, Dasylophia anguina: a, of body; b, of head; c, of prothoracic shield. B, Ceratosia tricolor: a, on body; b, on abdominal legs. C, Schizura ipomeæ: a, from third thoracic segment; b, from larva stage II; c, simple setæ from minute warts.
Glandular hairs and spines.—In some insects occur fine, minute, hollow setæ from which exude, perhaps through pore-canals of extreme fineness, droplets of a clear watery or plasma-like sticky fluid. The club-shaped tenent hairs of the feet of Collembola, and the hairs fringing the feet of Diptera, are modified glandular hairs. Here they serve to give out a sticky fluid enabling the insect to walk on smooth surfaces; they end in a vesicle-like bulbous expansion, which may contain numerous pore-canals. Those of caterpillars were first noticed by Zeller, and Dimmock has particularly described those of the larvæ of Pterophoridæ. They are either club-shaped, or variously forked at the end (Fig. 214, B, a). They are usually replaced after the first larval moult by ordinary, simple, solid, pointed setæ, and their use in caterpillars is as yet unknown. Whether these hairs, as seems most probable, arise from a specialized glandular hypodermal cell, or not, has not yet been discovered.
Fig. 215.—A, group of setæ arising from a subdorsal tubercle: cut, the cuticle; hy, the hypodermis; sc, the enlarged and specialized cells of the hypodermis which secrete the spines themselves; pglc, the nuclei which secrete the venomous fluid which fills the cavity of the seta (s), seen at p in a broken spine. B, a short entire, and a long broken seta (s-p); pgle, four poison cells; p, the poison in the hollow of the spine.
These temporary fine glandular hairs are probably the homologues of the larger true glandular bristles and spines of the later stages of certain lepidopterous larvæ, which are brightly colored and lead an exposed life, living through a large part of the summer. In these structures the bristles or spines are hollow, filled with a poisonous secretion formed in a single large, or several smaller specialized hypodermal cells situated under the base of the spine. In the venomous spines of Lagoa crispata the poisonous fluid in the larger spines (Figs. 215, C, 216, b) is secreted in several large cells situated at the base of the spine, and this is the usual form. In the finer spines of a large tubercle (Figs. 215, A, 216) there appears to be a differentiation of the hypodermal cells into two kinds, the large, basal deep-seated, setigenous cells (216, sc) and the poison-secreting nuclei (216, pglc) situated nearer the base of the setæ. The spines being filled with poison and breaking into bits in the skin of the hands or neck, cause great irritation and smarting. These nettling or poisonous hairs or spines are especially venomous in the larva of Orgyia, Empretia stimulea, Hyperchiria io, the larvæ of the saturnians (Fig. 217) and lasiocampids, etc. They rarely occur in insects of other orders, though the skin of Telephorus is said by Leydig to bear glandular hairs.
Fig. 216.—Section of a subdorsal tubercle from a larva in stage 1: sc, the setigenous cells, one for each seta; pglc, nuclei by which the poison is secreted; s, seta; p, poison in middle of a broken spine; cut, cuticle; sd, tub, spinulated surface of the subdorsal tubercle.
Leydig states that in the stout bristles of Saturnia there is, as in the integument of the body, a homogeneous cuticula, under which is the cellular matrix (hypodermis), and the clear contents (hyaloplasma) are secreted from the blood. The cell-structure of the hairs consist, as in the cells of the body, of spongioplasma and hyaloplasma. Leydig has observed the droplets of the secretion of the caterpillar of Saturnia carpini oozing through distinctly observable pores, and states that there are similar openings in the hairs and scales. Dewitz found easily observable openings at the end of the hair of a large exotic weevil (Fig. 130).
The advanced nymph of Psylla is also armed with clavate glandular hairs (Fig. 178).
Fig. 217.—Armature of last four segments of Callosamia promethea: a, a dorsal seta; b, one showing the poison (p) within.
The tubercles are outgrowths of the body-walls; they are either smooth, warty, or spiny, as in many caterpillars. While the armature of insects is of little morphological importance, it is evidently of great biological importance, the welfare or even the life of the insect depending upon it; and it varies in each species of insect, especially in Diptera, where the position of even a single seta characterizes the species.
Fig. 218.—Section through an antennal pectination of Saturnia carpini: a, hypodermis, formative cells of the hairs (c); d, cuticula; e, trachea.—After Semper.
Fig. 219.—Flattened hairs from the lateral tufts of larva of Gastropacha americana: A, three from the lateral tuft of Heteropacha rileyana.
The mode of development of the hairs was first described by Semper. In the pectination of the antenna of Saturnia carpini he observed that the hairs arise, like the scales of the wings, from large round formative-cells lying in the cavity, which send out through the hypodermis and cuticle a long slender process which finally becomes the hair (Fig. 218).
Tactile hairs are those setæ arising over nerve cells or nerve terminations and will be discussed under the organs of sense.
Fig. 220.—The same in G. quercifolia: a, a small hair ending in two minute processes.
Scales.—In very rare cases the hairs of caterpillars (Fig. 219) are flattened and scale-like, and this passage in the same insect of cylindrical hairs into flattened scale-like ones, shows that the scales are only modified hairs. Also, as we shall see farther on, Semper has proved that their mode of origin is identical. While true scales are characteristic of Synaptera (Thysanura and Colembola), as well as Lepidoptera and Trichoptera, they also occur in the Psocidæ (Amphientomum), in many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ, Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in the Culicidæ, and a few other Diptera, though they are especially characteristic of the Lepidoptera, not a species of this great order being known to be entirely destitute of them.
Fig. 221.—Flattened and spinulated hairs of tufts of larva of Acronycta hastulifera.
Fig. 222.—Scales from dorsal tuft, on second thoracic segment of larva of Gastropacha quercifolia.
The scales vary much in shape, but are more or less tile-like, attached to the surface of the body or wing by a short slender pedicel, and are more loosely connected with the integument than the hairs, which are thicker at the base or insertion than beyond.
The markings of the scales, both of Synaptera and Lepidoptera, are very elaborate, consisting of raised lines, ridges, or striæ with transverse ridges between. “The striæ of the transparent scales of Micropteryx are from about 500 to 300 to the millimetre, varying in different species. The opaque scales of Morpho, which show metallic reflections, have about 1400 striæ to the millimetre.” (Kellogg.)
The primary use of scales, as observed by Kellogg, is to protect the body, as seen in Synaptera and Lepidoptera. A nearly as important use is the production of colors and patterns of colors and markings, while in certain butterflies certain scales function as the external openings of dermal scent-glands, and they afford in some cases (as first claimed by Kettelhoit in 1860) generic and specific characters. Spuler has shown that the scales are strengthened by internal chitinous pillars. Burgess has observed in the scales of Danais plexippus that the under surface of the scales is usually smooth, or provided with few and poorly developed ridges, and this has been confirmed by Spuler and by Mayer (Fig. 226).
In the irised and metallic scales the ridges, says Spuler, are not divided into teeth, and they converge at the base to the pedicel and also toward the end of the scale (Micropteryx), or end in a single process beyond the middle (the brass-colored scales of Plusia chrysitis).
The arrangement of the scales on the wings is, in the generalized moths, irregular; in the more specialized forms they are arranged in bands forming groups, and in the most specialized Lepidoptera they are more thickly crowded, overlapping each other and inserted in regular rows crossing the wings, these rows either uniting with each other or running parallel. (Spuler.) The scattered irregular arrangement seen in Micropteryx is also characteristic of the Trichoptera and of Amphientomum.
Fig. 223.—Portion of a longitudinal section through one of the young pupal wings of a summer pupa of Vanessa antiopa: s, young scale; leu. cy., leucocyte; mbr. pr., ground membrane; prc, hypodermis-cells.
Fig. 224.—Portion of a longitudinal section through one wall only of the pupal wing of a specimen slightly older than that of Fig. 223; s, older scale.
Development of the scales.—The mode of origin of the scales was first worked out by Semper in 1886, who stated that in the wing of the pupal Sphinx and Saturnia they are seen, in sections, to arise from large roundish cells just under the hypodermis and which have a projection which passes out between the hypodermis (his “epidermis”) cells, expanding into a more or less spherical vesicle, the latter being the first indication of the future scale. He observed that the scales are not all formed at once, but arise one after another, so that on one and the same wing the scales are in different stages of development.
Fig. 225.—Portion of a longitudinal section through a pupal wing about eight days before emergence: s, formative scale-cell; upper s, a scale.
More recently Schaeffer has stated that the scales and also the hairs are evaginations of greatly enlarged hypodermis cells, and still more complete evidence has been afforded by A. G. Mayer (1896). In the wings of Lepidoptera, about three weeks before the imago emerges, certain of the hypodermis cells, which occur at regular intervals, begin to increase in size and to project slightly above the level of the hypodermis; these are Semper’s “formative cells,” and are destined to secrete the scales. They increase in length, and appear as in Fig. 223. In the next stage observed, the projections are much longer (Fig. 224). The hypodermis is now thrown up into a regular series of ridges, which run across the wing. Each ridge, says Mayer, corresponds in position with a row of formative cells, and each furrow with the interval between two adjacent rows. The scales always project from the tops of these ridges. The ground or basal membrane has not participated in this folding, and the deep processes of the hypodermis (prc) that once extended to this membrane have largely disappeared. Figure 225 represents a more advanced stage almost eight days before the emergence of the imago.
The scales are originally filled with protoplasm, which gradually withdraws, leaving behind it little chitinous bars or pillars which serve to bind together the upper and lower surfaces of the scales, and finally the scales become “merely little flattened hollow sacs containing only air.” As Mayer shows (Figs. 226, 227), from the study of scales examined four days before emergence of the butterfly (Danais), “the striations upon the upper surface of the scale are due to a series of parallel longitudinal ridges,” while the under side is usually smooth.
The mode of insertion is seen in Fig. 227. The narrow cylindrical pedicel of the scale is merely, according to Semper, inserted into a minute close-fitting socket, which perforates the wing-membrane, and not into a tube, as Landois supposed. Spuler describes a sort of double sac structure or follicle (Schuppenbalg) which receives the hollow pedicel of the scale. This was originally (1860) observed by F. J. Carl Mayer, but more fully examined by Spuler (Fig. 228) though not detected by A. G. Mayer.
Fig. 226.—Portion of a cross-section through the pupal wing of Danais plexippus, about six days before emergence: sg, scale; cta.al, wing-membrane; cl.frm, formative cell of the scale; mbr.pr, ground-membrane; fbr.h′drm, hypodermal fibres of pupal wings. A, portion of a longitudinal section through the pupal wing, eight or nine days before emergence; prc, processes of young hypodermis scales.—This and Figs. 223–225 after Mayer.
Spinules, hair-scales, hair-fields, and androconia.—Besides the scales, fine spinules occur on the thickened veins of the wings of the Blattidæ, where they resemble fir-cones; also in the Perlidæ, in the Trichoptera, and in the more generalized Lepidoptera (Micropterygidæ and Hepialidæ), occur, as indicated by Spuler, delicate chitinous hollow spinules scarcely one-tenth as long as, and more numerous than, the scales, which sometimes form what he calls “Haftfelds,” or holding areas. These spinules have also been noticed by Kellogg, and by myself in Micropteryx; Kellogg, and also Spuler, have observed them in certain Trichoptera (Hydropsyche). These also occur on the veins, and detached ones near large one-jointed hairs, or hair-scales, said by Kellogg to be striated. Kellogg has detected these scale-hairs, as he calls them, in Panorpa.
Fig. 227.—View looking down upon the upper (i.e. exposed) surface of one of the large scales situated on the veins of Danais plexippus, about four days before emergence: clm, chitinous pillars found in scales. A, a smaller scale, a, a′, sections of the scales. B, leucocyte found in the larger scale.—After Mayer.
Fig. 228.—Scale-follicles: A, of a scale of Galleria mellonella: r, neck-ring. B, the same of Polyommatus phlæas. C, the same of a hair on inner edge of hind wing of Lycæna alexis ♀.—After Spuler.
Fig. 229.—A, portion of wing of a caddis-fly (Mystacides). B, enlarged, showing the androconia and hair-scales. C, a separate androconium.—After Kellogg.
The “hair-scales” of the phylogenetically older Trichoptera correspond to certain scales of Lepidoptera, especially the Psychidæ (Spuler), variously called “plumules” (Deschamps), “battledore scales,” also certain minute cylindrical hairs. To these scent-scales is applied the term androconia. They are found, almost without exception, on the upper side of the fore wings, occurring in limited areas, such as the discal spots, or on folds of the wings. Fritz Müller has shown that they function as scent-scales, and are confined to the males. Kellogg has detected androconia-like scales on the wings of a caddis-fly, Mystacides punctata (Fig. 229).
Fig. 280.—Cross-section of androconia surface on wing of Thecla calanus; a, androconia; gl, gland of base; s, ordinary scales; w, wing in section.—After Thomas.
Thomas has proved by sections of the wing of Danais, etc., that the androconia arise from glands situated in a fold of the wing (Fig. 230), and he states that the material elaborated by the local glands, and distributed upon the surface of the wing by the androconia, is that which gives to many of the Lepidoptera their characteristic odor. On comparing these “glands,” it is evident that they are groups of specialized formative cells of Semper (trichogens), which secrete an odorous fluid, issuing perhaps from extremely fine pore-canals at the ends of the androconia. They thus correspond to the glandular hairs, poison-hairs, and spines of caterpillars, the formative cells of which contain either a clear lymph or poison.