Chapter VII: Letter XXX: States of Insects (2)

_Hymenoptera._--The larvæ of the different tribes of _Tenthredo_ L., almost the only Hymenopterous insects in which prolegs are present, have a variable number of these organs; some sixteen, as the saw-fly of the willow (_T. lutea_ L.), and this is the most numerous tribe of them, including the modern genera, _Cimbex_ F., _Pterophorus_, &c. Others have fourteen, as that of the cherry (_T. cerasi_ L.); and many others with only nine joints to their antennæ. A third class have only twelve, as that of the rose (_T. Rosæ_ L.), but this contains but few species. The last class contains those that have no prolegs at all, but only the six horny ones appended to the trunk. Of this tribe, the caterpillars of which have a very different aspect from the preceding, are those of the genus _Lyda_ F. (_T. crythrocephala_ L.)[310]. Two of the prolegs are anal, and the rest intermediate, and none are furnished with claws. This circumstance, in conjunction with the greater number of prolegs, except in the case of _Lyda_, will always serve as a mark to distinguish these _fausses chenilles_, as the French call the larvæ of saw-flies, from true _caterpillars_. The _dorsal_ prolegs of a species of _Cynips_ described by Reaumur have been before noticed.

_Coleoptera._--The larvæ of insects of this order are so little known or attended to, that no very accurate generalization of them in this respect is practicable. Many of them, in addition to their six horny legs, have a proleg at the anus; which in many cases appears to be the last segment of the abdomen, forming an obtuse angle with the remainder of it, so as to support that part of the body, and prevent it from trailing; and in some instances, as in _Chrysomela Populi_, a common beetle, secreting a slimy matter to fix itself[311]. In the larvæ of _Staphylinidæ_ this proleg is very long and cylindrical; in that of _Cicindela_ it is shorter, and in shape a truncated cone rather compressed; it is very short, also, in those of the _Silphæ_ that I have seen. In the wire-worm (_Elater Segetum_) it is a minute retractile tubercle, placed in a nearly semicircular space, shut in by the last _dorsal_ segment, which becomes also ventral at the anus. This space is in fact the last ventral segment. This seems characteristic of the genus[312]. From the underside of the body of the common meal-worm (_Tenebrio Molitor_), at the junction of the two last segments, when the animal walks, there issues a fleshy part, furnished below with two rather hard, long, and moveable pediform pieces, which the animal uses in walking[313]. In the larva of another beetle, whose ravages have been before noticed, under the name of the cadelle[314] (_Trogosita mauritanica_), a pair of prolegs are said to be found under the anal segment; and in that of the bloody-nose beetle (_Timarcha tenebricosa_), that segment is bifid. That of the weevil of the common water-hemlock (_Lixus paraplecticus_ F.) exhibits a singular anomaly: prolegs occupy the usual station of the true legs, being attached to the three segments representing the trunk[315]. This insect, however, does not appear to use them in moving. A pair in each of the twelve segments of the body are found in the grub of another weevil (_Hypera Rumicis_ Germ.), the nine last pair being the shortest, which all assist the insect in walking[316]. But the greatest number of prolegs is to be found in the Brazil subcortical larva lately mentioned. Besides the six horny legs of the trunk, this remarkable animal has four prolegs on each of the seven intermediate abdominal segments, and five on each side of the base of the last, making the whole number of prolegs, if so they may be called, amount to forty-four: a far greater number than is to be found in any larva at present known. When I wrote to you upon the motions of insects, I informed you that some larvæ moved by means of legs upon their back[317], but I was not then aware that any were furnished with them both on the back and the belly at the same time. By the kindness of Mr. Joseph Sparshall of Norwich, a very ardent and indefatigable entomologist, I am in possession of the larva of _Rhagium fasciatum_, a timber-feeding beetle. This animal on the ten intermediate segments of the underside of the body, which in the centre form a fleshy protuberance, has on it a double series of rasps, as it were, consisting each of two rows of oblique oblong prominences; and on the seven intermediate _dorsal_ segments there are also in the centre seven rasps of three or four rows each, of similar prominences: so that this animal at the same time can push itself along both by dorsal and ventral prolegs. It is worthy of observation, that a pair of these rasps is between the second and third pair of true legs.

_Diptera._--The larva of a little gnat, _Tipula stercoraria_ De Geer[318] (_Chironomus_ Meig.?), drags itself along by the assistance of a _single_ tubercle, placed on the underside of the first segment of the body, which the animal has the power of lengthening or contracting[319]. That of another beautiful _Chironomus_ (_C. plumosus_), remarkable for the feathered antennæ of the male[320], has _two_ short prolegs, or pediform but not retractile tentacula in the same situation[321]. Others, as that of _Tanypus maculatus_, &c. have _two pairs_, one attached to the anal and the other to the first segment[322]. _Tipula amphibia_ De Geer in this state has _ten_ prolegs, placed by pairs on the fourth, fifth, eighth, ninth, and tenth _dorsal_ segments[323]; and _Scæva Pyrastri_ F., one of the aphidivorous flies, has not fewer than _forty-two_, arranged in a sextuple series, seven in each row[324].

It may not be useless to close this long description of the _legs_ of larvæ with a tabular view of them, founded chiefly upon these organs; which afford very obvious marks of distinction.

I. Larvæ without legs.

i. With a corneous head of determinate shape (coleopterous and
hymenopterous _apods_--_Culicidæ_, some _Tipulidæ_, &c.
amongst the _Diptera_).

ii. With a membranaceous head of indeterminate shape (_Muscidæ_,
_Syrphidæ_, and other _Diptera_).

II. Larvæ with legs.

i. With legs only, and with or without an anal proleg
(_Neuroptera_, and many _Coleoptera_).

1. Joints short and conical (_Elater_, _Cerambycidæ_,
&c.).

2. Joints long and subfiliform (_Staphylinus_,
_Coccinella_, _Cicindela_, &c.).

ii. Prolegs only (many _Tipulidæ_, and some subcutaneous
Lepidopterous larvæ, &c.).

iii. Both legs and prolegs (_Lepidoptera_, _Tenthredinidæ_, and
some _Coleoptera_).

1. Without claws (_Tenthredinidæ_, &c.).

2. With claws (_Lepidoptera_, &c.).

I should next say something upon the spiracles, or breathing-pores, or any other external apparatus for the purpose of _respiration_, in larvæ; but I think it will be best to reserve the consideration of these for a subsequent Letter. We will therefore conclude this detailed description of their parts in their first state, with some account of their other.

iii. _Appendages._ The generality of larvæ have no other external organs than those already described; but in several of them we observe various kinds of retractile ones and others--protuberances--horn-like processes--rays, &c.; which, though not properly coming either under any of the above _parts_, or under the _clothing_ of these animals, yet require to be noticed. Upon these I shall now enlarge a little.

You must have observed upon the back of the last segment but one of the caterpillar of the silk-worm a horn-like process, rising at first nearly perpendicularly, and then bending forward. A similar horn, though confined in the genus _Bombyx_ to the silk-worm and a few others, if we may believe Madame Merian, who, however, often makes great mistakes, is found in the beautiful caterpillar of one of the largest and finest moths that we know (_Erebus Strix_[325]), the glory of the _Noctuidæ_, and in most of those of the hawk-moths (_Sphinx_ F.) [_S. Porcellus_, _Vitis_, and a few others excepted; in some of which, as _S. Labruscæ_, &c., this anal horn is replaced by a gibbosity, and in others, as _S. Œnotheræ_, by a callous eye-like plate[326]] in the same situation, but much longer[327], and commonly curving backwards over the tail[328]. Sometimes, however, as in _S. ocellata_ and _S. Stellatarum_, it is perfectly straight. These organs towards the apex are horny, and often end in a sharp point; nearer the base they are fleshy. They are without any true joint[329], yet the insect can elevate or depress them at pleasure. Under a lens, they usually appear covered with spinous eminences, arranged like scales. The use of these horns is quite unknown: Goedart fancies that they secrete a potent poison, and are intended as instruments of defence; but both suppositions are altogether unfounded. It has been remarked, that the body of those caterpillars which have these horns, is firmer, and yields less to the touch than that of those which have no such appendages[330]. The larva of a small timber-devouring beetle (_Lymexylon dermestoides_ F.) has, like the above caterpillars, a long horn, and in the same situation: it has also a singular protuberance on the first segment[331]. Upon some other caterpillars, as in _Bombyx Stigma_ F., a singular pair of horn-like appendages arises from the back of the second segment of the body, excluding the head. In a tawny-coloured one from Georgia, with a transverse row of short black spines on each segment, these horns are half an inch long, black, covered with spinous eminences, rather thickest at the base, and terminate in a little knob. They appear to articulate with the body at the lower extremity. I have another species, black, with narrow longitudinal yellow stripes, in which these horns are of equal thickness at base and apex, but with the same terminal knob. _Danais Archippus_ has a pair of tentacula at the head, and another pair, but shorter, at the tail; and _D. Gylippus_ has, besides these, two in the middle of the body[332].

We are equally ignorant of the use of the upright horn found upon the back of the fourth segment in the larva of some moths (_Noctua Psi_, and _tridens_ F.) which is of a construction quite different from that of those last described. It is cylindrical, slightly thinner at the apex, which is obtuse, fleshy, incapable of motion, of a black colour, and about two lines long. On the same segment, also, in the case-worms (_Trichoptera_ K.) are three fleshy conical eminences, which the animal can inflate or depress, so that they sometimes totally disappear, and then in an instant swell out again. When retracted, they form a tunnel-shaped cavity, varying in depth[333]. Reaumur conjectured that these eminences were connected with respiration, and one circumstance seems in favour of this conjecture, that this segment has not the respiratory threads observable in the subsequent ones. Latreille mentions certain fleshy naked eminences placed upon the ninth and tenth segments of some hairy caterpillars, which, like those just mentioned, the animal can elevate more or less. They are often little cones; but when it would shorten them, the summit is drawn in, and a tunnel appears where before there was a pyramid[334].

In a former Letter I gave you a short account of the remarkable Y-shaped, as it should seem, scent-organs (_Osmateria_) of the beautiful caterpillar of the swallow-tailed butterfly (_Papilio Machaon_ L.), and others of the _Equites_[335]; I will now speak of them more fully. That found in the former is situated at the anterior margin at the back of the first segment, close to the head, from which at first view it seems to proceed. At the bottom it is simple, but divides towards the middle, like the letter Y, into two forks, of a fleshy substance[336], which it can lengthen, as a snail does its horns, to five times their ordinary extent, or retract them within the stalk, so as wholly to conceal them. Sometimes it protrudes one fork, keeping the other retracted; and often withdraws the whole apparatus for hours together under the skin, and its place is only marked by two tawny-coloured dots, so that an ordinary spectator would not suspect the existence of such an instrument[337]. Unfortunately this larva is rare in this country, so that I can scarcely flatter you with the hope of seeing this curious organ in a living specimen[338], unless you choose to import a parcel of its eggs from the south of Europe, where it is common. This you will think rather a wild proposition; but why should not Entomologists import the eggs of rare insects, as well as botanists the seeds of rare plants? But if you will be satisfied with the dissection of a dead specimen, I have several, done by the ingenious Mr. Abbott of Georgia, in which this part is well exhibited[339].

Another small caterpillar, as it should seem, of a geometer, prepared by the same gentleman, exhibits a pair of similar horns on the fifth and sixth segments: in these the common base from which the fork proceeds is very short and wide, and each branch grows gradually more slender from the base to the apex, where it is involute. Whether these are retractile, or whether they correspond with those of _P. Machaon_ in their nature and use, cannot be ascertained from a dead specimen: as they belong to a larva of a quite different tribe of _Lepidoptera_, the probability is, that they essentially differ. Two globose retractile vesicles issue from the ninth and tenth segments of those of _Arctia chrysorrhea_, &c.[340]

A great number of Lepidopterous larvæ, particularly those which are smooth and of a moderate size, have between the under-lip and fore-legs a slender transverse opening, containing a teat-like protuberance of the same construction as the furcate horn of the caterpillar of the beautiful mountain-butterfly, _Parnassius Apollo_; and, like that, can either be wholly retracted and concealed, or by pressure be extended to the length of one of the legs. In some larvæ this part is of a subhemispherical figure, generally single, but sometimes double. It is commonly, however, more slender and conical; and when of this shape, it is sometimes quadruple[341]. The use of this part is not very clearly known: some have supposed it to be a second spinneret, and to be of use in fabricating the cocoon; but it is more probable that it secretes some other kind of fluid, and is connected with defence.

The singular organ in a similar situation, evidently for that purpose, with which the puss-moth endeavours to annoy its assailants, has been described in a former Letter, to which I refer you[342]. Bonnet, who was the first that discovered this organ, ascertained that it might be cut off without injury to either larva or imago. He also remarked in a caterpillar found in the wild succory (_Cichorium Intybus_) another short, black, needle-shaped organ between the conical part just described and the under-lip[343]. De Geer mentions a remarkable fleshy horn-like style, which issues from the lower side of the first segment, between the head and the legs of the case-worms (_Trichoptera_): he does not describe it as retractile, or it might be regarded as analogous to those of _Lepidoptera_ similarly situated, that I have just noticed[344]. In that of the emperor-moth (_Saturnia Pavonia_), there are perforated tubercles, which when the animal is molested spirt forth a transparent fluid[345].

The horn-like appendage of the puss-moth (_Cerura Vinula_) is situated at the tail of the insect, and is composed of two distinct cylindrical diverging branches, each about four lines long, not united at the base. Each of these is hollow, and includes a smaller cylindrical piece, which can be protruded at pleasure, and withdrawn again, as a pencil within its case; or, rather, as the horns of a snail. The two outer horns are tolerably firm, moveable at their base, and beset with black spines; the interior tentacula are fleshy, moveable in every direction, and in full-grown larvæ of a rose colour. The animal seldom protrudes them, unless in some way disturbed; and frequently it approximates the two outer cases so closely that they resemble a single horn. It appears to use these inner horns, when protruded, as a kind of whip to drive away the flies, especially the Ichneumons, that alight upon its body. When touched in any place, it will unsheath one of them, and sometimes both, and with them strike the place where it is incommoded[346]. A similar organ is found in some other _Bombycidæ_, as _B. Tau_ and _Furcula_ F. Reaumur mentions a caterpillar that to this kind of tail added the resemblance of two ears, or two cylindrical bodies, terminating in a point, which emerged from the first segment behind the head[347]. In another observed by the same author, the legs were replaced by a single horn, but which did not appear to send forth an internal one: from the back of its fourth segment also emerged a single conical or pyramidal fleshy eminence or cleft, terminating in two points[348]. Some of the tropical butterflies also, as may be seen in the figures of Madame Merian, have two diverging anal horns instead of anal prolegs; but it does not appear that they incase tentacula[349]. Wherever these caudal horns are found, the above prolegs are wanting[350]. Two conical anal horns also distinguish the caterpillar of one of the moths called _Prominents_, _Notodonta camelina_; but these are not terminal, but on the back of the last segment but one[351]. In that of another British moth, _N. ziczac_ F., there are three dorsal prominences, one near the anus, and two more in the middle[352]. Some Geometers (_G. fuliginosa_, &c.) have two erect horns on the eleventh segment, and others (_G. syringaria_, &c.) two recurved ones on the eighth[353]. I must not here omit to mention the curious hooks emerging from two tubercles on the back of the eighth segment of the ferocious larva of that beautiful tiger-beetle, the _Cicindela campestris_ L., not uncommon on warm sunny banks. This animal with incessant labour, as we are informed by M. Desmarets, digs a cylindrical burrow, to the enormous depth, the size of the animal considered, of eighteen inches. To effect this, it carries out small masses of earth upon its large concave head; and having often occasion to rest in ascending this height, by means of these hooks[354] it fixes itself to the sides of its burrow, and, having finally arrived at its mouth, casts off its burthen. When these insects lie in wait for their prey, their head, probably in conjunction with the first segment of the body, accurately stops the mouth of the burrow, so as to form an exact level with the surrounding soil; and thus careless insects, walking over it without perceiving the snare, are seized in a moment and devoured[355].

Another kind of appendage, which is found in some larvæ, is the organ employed by them to carry the excrement; with which, instead of letting it fall to the ground, they form a kind of umbrella to shelter and probably conceal them. All the tortoise-beetles (_Cassida_ L.) have instruments for this purpose, as well as an Indian genus (_Imatidium_ Latr.) very nearly related to them. This instrument is a kind of fork, half as long as the body, consisting of two branches, growing gradually smaller from the base to the summit, where they terminate in a very fine point, of a substance rather horny, and attached to the body near the anal orifice. They are armed on the outside with short spines, from the base for about a third of their length. When this fork, as it usually is, is laid parallel to the back, with its points towards the head, the anal aperture points the same way. When the animal walks, the fork points the other way, and is in the same line with the body, and the anus assumes a prone position[356].

The larvæ of a genus of flies (_Volucella_ Geoffr.) remarkable for inhabiting the nests of humble bees, are distinguished on their upper side by six long, diverging, pointed, membranous radii; placed in a semicircle round the anus[357]: what the particular use of these organs may be, has not been conjectured. Another in my collection has only four upper radii, but below the anus are two fleshy filiform tentacula. One of a Tipulidan described by Reaumur, has also four upper teeth; but instead of two subanal tentacula, has six[358]. The singular larva of another of this tribe (_Chironomus plumosus_) has on the two last segments four long, fleshy, filiform, flexible tentacula, often interlaced with each other; which, according to the same illustrious author, are used by the animal to fix its caudal extremity, like the geometers, that the other end may be at liberty. Besides these organs round the anus, it has also four other oval ones, of uncertain use: not to mention the two prolegs, which M. Latreille thinks are air-tubes[359]. Jointed anal organs are observable in other larvæ: those in that of a saw-fly described by De Geer (_Lyda_ F.) consist of three joints[360]; in that of _Hister cadaverinus_, a carnivorous beetle, of two[361]. The larva as well as the pupa and imago of _Ephemera_ is furnished with three long diverging multiarticulate tails, which are probably useful as a kind of rudder to assist and direct their motions. That of the smaller dragon flies (_Agrion_ F.) is furnished with three long vertical laminæ, by moving which, as fish do their tails, from side to side, the animal makes its way in the water[362]. That singular one, also, with a hooked head, figured by Reaumur, has a single swimming lamina, or fin, shaped like a fan, and placed in a vertical position under the tail[363].

The whole circumference of the body in some coleopterous larvæ,--for instance, in that of the tortoise-beetle lately mentioned,--is surrounded with appendages like rays. These are sometimes simple, rough with very short spinous points[364]; but I have a dipterous larva, in which these radii themselves are beautifully pinnated by a fringe of longish spines on each side. Reaumur has described the grub of a beetle, the genus of which is uncertain, and which feeds upon the larva of _Aleyrodes Proletella_, whose body is margined on each side by eight triangular fleshy mammular processes, terminating each in a bristle, which give it a remarkable aspect[365]. The curious scent-organs with which the larva of _Chrysomela Populi_ is fringed have been before fully described; and therefore I shall only mention them here[366].

In the larvæ of the lace-winged flies (_Hemerobius_), and ant-lions (_Myrmeleon_), the anus is furnished with a small fleshy retractile cylinder, from which proceeds the silken thread that forms the cocoon inclosing the pupa[367]. Providence has many different ways of performing the same operation. From the structure of the oral organs of these animals, the silk could not conveniently be furnished by the mouth; the Allwise Creator has therefore instructed and fitted them to render it by a spinneret at the other extremity of the body.

The respiratory anal appendages of many Dipterous larvæ will be fully described in a subsequent Letter: I shall therefore now only further observe upon this subject, that although there is seldom any alteration in the form of these appendages &c. in the same species, the caterpillars of two moths (_Cerura Vinula_ and _Attacus Tau_), however, are exceptions. The former, when young, has two hairy projecting ear-like protuberances, which it entirely loses, as I have myself observed, before it assumes the pupa; and the latter, in like manner, after its third change of skin, is deprived of its bent thorn-like points which attend it when young[368]. It is remarkable that these last larvæ, when just excluded from the egg, are also entirely destitute of these appendages; they soon, however, appear, from slight elevations which mark their situation, and rapidly acquire their usual form[369]. Changes of a similar kind, hitherto unobserved, may probably take place in other species.

iii. _Figure._ I am next to consider the general figure or shape of larvæ. All of them, with but few exceptions[370], agree in having a body more or less constricted at intervals into a series of rings or _segments_; usually in number, _twelve_; often nearly equal in length, but sometimes in this respect very dissimilar[371]. The general outline or shape of the body is extremely various: most frequently it approaches to cylindrical, as in most of the caterpillars of _Lepidoptera_, and of the Hymenopterous tribe of saw-flies (_Tenthredo_ L.). The next most common figure is that more or less oblong or oval one, sometimes approaching to conical, found in many of the larvæ usually called grubs; such as those of the weevil (_Curculio_ L.) and of the capricorn (_Cerambyx_ L.), and other coleopterous tribes; of bees, and all Hymenopterous insects but the saw-flies; and also of a large number of flies (_Diptera_). In some the figure approaches to fusiform, as in most of the moths of the Fabrician genus _Lithosia_. In others, as in those of the water-beetles (_Dytiscus_, &c.), it approaches to an obovate shape, being widest towards the head, and terminating in a point at the anus. In others, again, it is linear; an example of which is that of _Staphylinus_. Some are convex, and others gibbous, above, and flat underneath; as those of _Silpha_, _Chrysomela_ and many other beetles. Others are flat, both above and below, and depressed like a leaf; a remarkable instance of which has been before noticed[372]. Some are very long, as those of most _Lepidoptera_; others very short, as that of the ant-lion (_Myrmeleon_). Many other peculiarities of form in individuals might be instanced; but a dry enumeration of these would be of no great use to you. They can only be advantageously learned by the study of good figures, and by watching the actual metamorphosis of the singularly-formed larvæ that you meet with.

Instead, therefore, of any further specification of individual forms, I shall now endeavour to give you, as far as my own knowledge of them and the information I can collect from other sources will enable me, a larger and more general view of the kinds of larvæ; for analytical inquiries lose half their value and importance unless we proceed to apply them synthetically, by forming, if possible, into groups the objects with which we are individually acquainted.

Partial attempts at a synthetical arrangement with regard to the larvæ of _Lepidoptera_ and the saw-flies (_Tenthredo_ L.) have been made both by Reaumur and De Geer. M. Latreille also has recently given a _Tableau méthodique et général_ of articulated animals furnished with jointed legs, considered in their first state[373]. The former of these is chiefly founded upon the number of the prolegs, and the latter upon the metamorphosis, prolegs, habits, head, and parts of the mouth, without any other notice of the configuration. Mr. Wm. MacLeay, who, though young in years, is old in science and critical acumen, has started a perfectly new hypothesis upon this subject. In the progress of his inquiries into the natural arrangement of animals, particularly of _insects_ in the _Linnean_ sense, he has been the first to observe, that the relation which organized objects bear to each other is of two kinds; one of real _affinity_, and the other only of _analogy_, or resemblance. This important distinction, upon which I shall enlarge in a future Letter, when I come to treat of Systems of Entomology, he has applied, in a way quite original, to larvæ in general, but more particularly to those of the _Coleoptera_ order. For the basis of his system he assumes a relation of analogy between the _larvæ_ of Insects that in the progress of their metamorphosis assume wings, and those that do not, which form his class _Ametabola_, so that the prototypes of the former shall be found amongst the latter[374]. But though Mr. MacLeay appears to consider the analogy between these two as _primary_, he extends it in a _secondary_ sense to the _Crustacea_, at least in several instances[375]. Upon this occasion he very judiciously remarks, that "in terming larvæ _Chilognathiformes_ or _Chilopodiformes_, it is not meant that they are _Scolopendræ_ or _Iuli_, or even near to them in affinity; but only that they are so constructed that certain analogical circumstances attending them strongly remind us of these _Ametabola_[376]." This remark you will bear in mind while I am treating of this subject. It should seem from another part of the same paragraph, that the comparison which our learned Physiologist recommends, is between the _young_ of the _Ametabola_ and the larvæ of the corresponding groups of _Coleoptera_. This must be understood to refer chiefly to the young of the _Chilopoda_ and _Chilognatha_, since they approach nearer to them in that state, having then only six legs; but the rest of the _Ametabola_ should certainly be brought to this comparison in their adult state: and even the two former orders in that state more strongly resemble numerous coleopterous larvæ, than when they are young and much shorter. I before called your attention to the remarkable circumstance that contrasts very many larvæ of Hexapod insects that become winged in their perfect state with adult _Myriapoda_: namely, that in one the progress to this state is by losing their prolegs and shortening their body; while in the other, the reverse of this takes place, numerous prolegs and additional segments being gained before they arrive at maturity[377]. As the multiplication of organs is a sign of imperfection, it may be affirmed of the former of these tribes, that their progress is towards greater perfection; while that of the other may be called a degradation. As larvæ may be regarded as a stepping-stone by which approach is made from the apterous to the winged tribes of Insects, it seems most consistent with general analogy that each should connect with the other in that state in which the resemblance is greatest. Now the _Myriapoda_ resemble larvæ, as we have just seen, most when in their _adult_ state; therefore the comparison should be between larvæ and adult _Myriapoda_.

Mr. MacLeay divides _coleopterous_ larvæ into five tribes thus characterized:--

1. _A carnivorous hexapod larva, with an elongate linear flattened body, having a large head armed with two sharp falciform mandibles, and furnished with six granular eyes on each side._ This kind he denominates _Chilopodiform_, as having for its type in the _Ametabola_, _Scolopendra_ L. The examples he gives are _Carabus_ and _Dytiscus_.

2. _A herbivorous hexapod larva, with a long and almost cylindrical body, so fashioned that the posterior extremity being curved under the breast, the animal when at rest necessarily lies like an IULUS on its side._ This tribe he denominates _Chilognathiform_, from _Iulus_ L. His examples are, the larvæ of _Petalocerous_ insects, as _Scarabæus_ L., _Lucanus_ L. &c.

3. _An apod larva, having scarcely the rudiments of antennæ, but which is furnished instead of feet with fat fleshy tubercles; which, when continued along the back and belly, give the animal a facility of moving in whatever way it may be placed._ These he denominates _Vermiform_, from certain of the _Vermes intestina_ and _Mollusca_ of Linné which he has associated with his _Annulosa_[378]. His examples are, _Curculio_ L. and _Cerambyx_ L.

4. _A hexapod and distinctly antenniferous larva, with a subovate rather conical body, of which the second segment is longer and of a different form from the others, so as to give the appearance of a thorax._ His denomination for these is _Anopluriform_, from _Pediculus_ L., forming Dr. Leach's _Anoplura_. His examples are, _Coccinella_ and _Chrysomela_ L.

5. _A hexapod antenniferous larva of an oblong form, having like the former vestiges of a thorax, besides two or more articulated or inarticulated setaceous or corneous appendages to the last segment of the abdomen._ This tribe he calls _Thysanuriform_, from _Lepisma_ and _Podura_ L., forming M. Latreille's order _Thysanura_. His example is _Meloe_ with a note of interrogation[379].

The system here stated, of naming and characterizing larvæ from the resemblance and analogy, in many cases very striking, that they bear to the apterous tribes, is a very happy and original one, and does its author great credit; yet I think in some instances, as I shall soon have occasion to point out to you, the application of it is not so happy as the first idea. But this is always the case when a new law of nature is discovered; the proper application of it is gradually developed, and it does not at all detract from the merit of the first discoverer, that all the bearings of such law do not strike him as it were intuitively.

Having, however, got the vantage-ground afforded by this discovery of my friend, let us see if by standing upon it we cannot get a tolerable generalization of the larvæ of all orders of insects that undergo a metamorphosis. But first I must observe, that as in the perfect animals, so in their larvæ, the different groups are connected by certain _transition_ species, exhibiting characters common to two or more of them; and likewise that in many cases, which you will see as we proceed, the analogy is as strong or stronger between them and the _Crustacea_ (and in a few instances _Arachnida_, and even _Mollusca_) than the _Ametabola_. My denominations, therefore, will be taken from those tribes where the analogy appears to me the most striking, and not from the _Ametabola_ alone.

I shall begin by drawing up for you a list of the Primary forms that I seem to have observed, and their characters; and then going through the orders, shall give you the examples of each, with such observations upon them as the case may require.

_Primary Forms of Larvæ._

APTERA. ARACHNIDA. CRUSTACEA.

ANOPLURIFORM. ARANEIDIFORM. ISOPODIFORM.
THYSANURIFORM. ONISCIFORM.
CHILOPODIFORM. IDOTEIFORM.
CHILOGNATHIFORM. AMPHIPODIFORM.
STOMAPODIFORM.
MOLLUSCA. ANNELIDA. DECAPODIFORM.
BRANCHIOPODIFORM.
LIMACIFORM. VERMIFORM.

_Characters._

1. _Anopluriform._ Carnivorous; hexapod; antenniferous:
with a shortish oblong depressed body, and distinct
thoracic shield. Example: _Psocus_, _Coccinella_, most
_Hemiptera_[380].

2. _Thysanuriform._ Polyphagous; hexapod; antenniferous: body
with segments of trunk distinctly marked; anus often
furnished with setæ or mucro. Ex. _Meloe_[381]? _Thrips_,
_Aphis_.

3. _Chilopodiform._ Carnivorous; subhexapod; antenniferous: body
depressed, elongate, linear, with falcate acute mandibles,
a distinct thoracic shield, and an anal proleg. Ex.
_Cicindela_[382]? _Carabus_ L.

4. _Chilognathiform._ Herbivorous; body subcylindrical,
elongate, linear; no thoracic shield; often many prolegs,
sometimes a retractile one, and sometimes none.--Ex.
_Elater_, _Petalocera_, most _Lepidoptera_, _Tenthredo_
L.[383]

5. _Vermiform._ Polyphagous; apod or hexapod: with very short
legs; antennæ nearly obsolete; body fleshy, plicate, with
sides often plicato-papillose; no distinct thoracic shield.
Ex. _Curculio_ L., _Cerambyx_ L., _Musca_ L., and many other
_Diptera_[384].

6. _Araneidiform._ Carnivorous; hexapod: body very short;
mandibles long, suctorious; animal lying in wait for its
prey in a pitfall it has prepared; motion retrogressive. Ex.
_Myrmeleon_ L.[385] _Cicindela_?

7. _Isopodiform._ Saprophagous; hexapod; antenniferous, with
longer antennæ: body oblong; thoracic shield distinct; anus
styliferous or laminiferous. Ex. _Blatta_ L. _Silpha_ L.?

8. _Onisciform._ Herbivorous; polypod; antenniferous: body
short, oblong, depressed, margined. Ex. _Erycina_, _Lycæna_,
in the _Lepidoptera_, and some species of _Tenthredo_ L.[386]

9. _Idoteiform._ Subcortical; hexapod; antennæ obsolete: body
much depressed, with the last segment elongate, terminating
in three or more mucros. Ex. Larva from Brazil. Perfect
insect at present unknown.

I have placed this larva, which was described above[387], amongst crustaceous forms, because of the remarkable resemblance which the last segment of the body bears to that of the _Idoteidæ_; but I do it with considerable hesitation, since in other respects its type seems to be in the _Ametabola_. In its want of antennæ, very short legs, and ventral asperities, it resembles some of the _Vermiform_ larvæ; in its small head, distinct thoracic shield, and oblong shape, it approaches the _Anopluriform_; and in its very depressed body, but not at all in other respects, the _Chilopodiform_. At any rate, it appears of a primary Type.

10. _Læmodipodiform._ Herbivorous; hexapod; antenniferous, with
long antennæ: body elongate, subcylindrical; second segment
of the trunk the longest; anterior pair of legs distant from
the other two. Ex. _Phasma_.

11. _Amphipodiform._ Herbivorous; hexapod; antenniferous, with
long antennæ: body shortish, compressed, saltatorious. Ex.
_Gryllus_ L.[388]

12. _Stomapodiform._ Carnivorous or saprophagous; hexapod;
antenniferous, with long antennæ: body elongate,
subdepressed, with raptorious hands, and abdomen wider than
the trunk; in aquatic species furnished with lateral gills.
Ex. _Mantis_, _Ephemera? Sialis?_

13. _Decapodiform._ Carnivorous; hexapod; antenniferous: body
elongate, narrow, convex, compressed, tapering towards the
tail; tail with natatorious laminæ. Ex. _Dytiscus_ L.,
_Agrion_ F.[389]

14. _Branchiopodiform._ Carnivorous?; aquatic; apod;
antenniferous: head distinct: body transparent, flexile,
furnished with a respiratory tube just above the tail. Ex.
_Culex_[390].

15. _Limaciform._ Herbivorous; apod, or with very short legs:
body ovate or obovate, slimy. Ex. _Apoda_ Haw., _Tenthredo
Cerasi_ L.

The above are the principal forms that appear to me _Primary_ (though some doubt may rest upon the ninth and tenth); and probably others will hereafter be discovered, since at present our knowledge of the larvæ of most of the Orders is very limited. And now having given you this generalization of them, as far as they are known to me, I shall next, in a slight survey of those of the different orders, lay before you what I have further to observe upon this subject.

_Coleoptera._ The _Anopluriform_ coleopterous larvæ, according to Mr. W. MacLeay's view of them, include both those of _Coccinella_ L., _Chrysomela_ L., and _Cassida_ L.; but this appears to me to admit of further consideration. With regard to the two former--those of _Coccinella_ are carnivorous, those of _Chrysomela_ herbivorous; the first is also usually more flat and depressed. As to the latter, _Cassida_[391], it seems to me to belong to a peculiar type, distinguished not only by its radiated margin, but by the remarkable deflected anal fork on which it carries its excrement. At present I know no analogous form amongst the apterous tribes; I must therefore leave this without a denomination. Perhaps the larva of _Hispa_ or _Alurnus_, when known, will throw light upon this subject. The larva of _Endomychus_ agrees with that of _Coccinella_.

There are very few known larvæ that approach to a true _Thysanuriform_ type in this order: that most celebrated is the one supposed to belong to _Meloe_; but the claim of this to be so considered, is, as we have seen, rather dubious. Should this point at last be satisfactorily ascertained, it will probably carry with it the whole tribe of vesicatory beetles. But even this animal in its _general_ structure is _anopluriform_: the only circumstance that gives it any analogy to the _Thysanura_ being its anal setæ. Mr. William MacLeay is inclined to regard some of the larvæ of the _Malacodermi_ Latr., but which of them he does not state, as probably belonging to the tribe in question[392]. Those of _Lampyris_ and of _Telephorus_, as described and figured by De Geer[393], appear to me intermediate between the _Anopluriform_ and _Chilopodiform_ Types: they have no anal setiform or styliform appendages, their mandibulæ are falcate, and their habits seem carnivorous.

Examples of _Chilopodiform_ coleopterous larvæ are more numerous. Of this description are those of _Gyrinus_, _Cicindela_, _Carabus_, and _Staphylinus_. That of the first, indeed, appears to be the most perfectly _Scolopendriform_ of any yet known; yet the gills or respiratory laminæ, a pair of which issues from each abdominal segment, and two pair from the last[394], prove that there is no slight analogy between it, and indeed many other aquatic larvæ, and the _Stomapoda_ amongst the _Crustacea_. A remarkable instance of analogy with the _Decapoda_ of the same _Class_ is presented by the larva of _Dytiscus_, &c. which Mr. MacLeay considers as _Chilopodiform_, but which exhibits no other resemblance to _Scolopendræ_ than in its predaceous habits and threatening aspect. Its convex, compressed, tapering body, terminating in setæ or laminæ, is certainly much more like that of a shrimp or a prawn; to which the older Entomologists thought it was akin[395], and after which they named it. As Mr. MacLeay's object was, to take all his forms from the _Ametabola_, perhaps these larvæ will best fall in with his _Chilopodiform_ type; though in the general form of their body they most represent a section (_Lepisma_ L.) of the _Thysanura_.

_Chilognathiform_ forms are equally numerous in the Coleoptera with the preceding. The wire-worm, or larva of _Elater Segetis_, as to shape best represents the full-grown _Iulus_[396], and those of the _Petalocera_ (_Scarabæus_ L., _Lucanus_ L.) the young one.

The most abundant of all forms in this order, is, I think, the _Vermiform_, upon which I have nothing further to remark.

With regard to _Crustaceous_ forms in _Coleoptera_, besides the _Decapodiform_ just noticed, I possess two specimens of larvæ of _Silphidæ_ which seem to exhibit a considerable analogy with the _Isopodous Crustacea_, one rather convex and the other flatter, so as to give the idea of an _Armadillo_ and of an _Oniscus_.

_Strepsiptera._ Larva _Vermiform_.

_Dermaptera._ Larva _Thysanuriform_. Type _Podura_ or _Sminthurus_.

_Orthoptera._ Mr. MacLeay considers the larvæ of this Order as primarily _Thysanuriform_[397], though he allows the resemblance between them and _Amphipoda_ to be particularly striking[398]. For my own part, their prototypes appear to me to be in the _Crustacea_, and their analogical relations to the _Thysanura_ much more distant. I trust this will appear to you the reverse of dubious in a progress through the Crustaceous Orders. I begin with the _Isopoda_. Take the larva of a _Blatta_, and place it between a _Lepisma_, or _Machilis_, and an _Oniscus_, or _Porcellio_; you will find that in shape and width, and the form of its anal styles, it resembles the latter much more than it does the former, with which it possesses scarcely any character in common, except its multiarticulate antennæ. It is remarkable, that amongst the _Blattidæ_ we meet with species that represent both the _Oniscidæ_ and _Armadillo_ or _Glomeris_[399], the latter being more convex than the former. In their habits the Blattæ certainly agree with _Lepisma_; and Dumeril, who thought the latter and _Podura_ subject to a metamorphosis, imagined they were related[400].

The Spectres of Stoll (_Phasma_ F.) are so strikingly analogous to another crustaceous tribe, the _Læmodipoda_, particularly the genus _Caprella_, that Montagu gave one species the Trivial name of _Phasma_[401]. The jumping _Amphipodiform_ Crustacea are represented extremely well by _Gryllus_ L., and the _Stomapodiform_, particularly _Squilla Mantis_, by _Mantis_. The resemblance in this last instance is so very striking, that it cannot escape the eye of the least intelligent observer. Orthopterous insects may perhaps one day be discovered analogous to the two other crustaceous orders, the _Decapods_ and _Branchiopods_; but at present I know of none of that description.

_Hemiptera._ The larvæ of this order, which in general resemble the perfect insect, except that they have no wings, seem most commonly to belong to the _Anopluriform_ type[402]; but the _Aphides_, _Chermes_, and _Thrips_ may, I think, be regarded as more analogous to the genera _Podura_ and _Sminthurus_ in the _Thysanura_[403]. I have some suspicion that the _Nepidæ_, _Naucoris_, and the remipedes, _Notonecta_, _Sigara_, &c. may find their prototypes amongst the _Crustacea_; but my confined knowledge of the latter does not enable me to point to any individual genera or tribes that they may be presumed to represent.

_Neuroptera._ As the kinds of larvæ of the different tribes composing this order, as it now stands, are very various, it is to be expected that the analogical forms they represent are equally so. The _Libellulina_ MacLeay (whose metamorphosis that gentleman has denominated _subsemicomplete_, a term warranted by their losing in their perfect state the mask before described[404]) in their oral organs, particularly by their galeate _maxilla_ and distinct _ligula_[405], have some relation to the _Orthoptera_, the prototypes of whose larvæ we have found amongst the _Crustacea_: probably, therefore, those of the tribe in question lurk in that class; a suspicion that receives strong confirmation from the larva of _Agrion_[406], which in its tapering body and anal natatorious laminæ represents a shrimp. The larvæ of that very peculiar and distinct tribe, the _Ephemerina_, appear to be intermediate between the _Stomapodiform_ and _Thysanuriform_ types. Their natatory respiratory abdominal laminæ seem copied from the former, and their anal diverging setæ from the latter[407]. The _Myrmeleonina_, as well in their general form as in their motions and habits, present a most singular analogy with the tribe of spiders, as does also in some respect that of _Cicindela_. With regard to _Panorpa_, which Mr. MacLeay remarks is related to _Myrmeleon_[408], and is a most ferocious insect[409], as its larva has not yet been discovered, nothing certain respecting its analogical form can be asserted; but should it, like the male fly, represent the scorpion, both orders of _Arachnida_ will have their representatives in the class we are considering. The _Corydalina_, as far as the larva of _Hemerobius_ instructs us, is _Chilopodiform_, but with a tendency to the _Araneidiform_ Type. The _Ametabola_ also furnish the prototype of the next tribe, the _Termitina_, which, as is evident both from _Psocus_ and _Termes_, are perfectly _Anopluriform_. The _Sialina_, or _Plicipennes_ of Latreille, excluding _Trichoptera_ Kirby, appear to me to be intermediate between the _Chilopodiform_ and _Stomapodiform_ Types, and not without some relation to the _Thysanuriform_. Their pediform, jointed, respiratory abdominal appendages, their head and falcate mandibles, seem copied from the first tribe. The same appendages considered as organs of respiration, and their taper forks, are moulded upon the plan of the _Stomapodiform Crustacea_, and the long seta which terminates the abdomen is upon the _Thysanuriform_ plan[410].

_Trichoptera._ The larvæ of this order appear also to be constructed upon a double plan. The respiratory threads observable in both the upper and under sides of the abdomen connect it with the _Stomapoda_, and its cylindrical elongate body with _Chilognathiform_ types in the _Lepidoptera_[411].

_Lepidoptera._ The great majority of larvæ in this order are _Chilognathiform_, but there are exceptions to this remark. Those of the _Geometræ_ recede from this type, both in their motions and the distance and number of their legs. In both these respects they represent the _Læmodipoda_ in the _Crustacea_[412]. Other caterpillars are _Onisciform_; and a third sort seem to leave the _Annulose_ type, and imitate that of the _Mollusca_, and one is figured by Madame Merian[413] which appears to tend even to the _Chilopodiform_ type.

_Hymenoptera._ In this order the larvæ of the saw-flies, _Tenthredo_ L., are in general _Chilognathiform_, though some are _Onisciform_, others _Limaciform_, and those of _Lyda_ F. (_Cephaleia_ Jur.) and _Sirex_[414] have a _Vermiform_ tendency, and are a stepping-stone to those of the rest of the order, which are all _Vermiform_ and apods.

_Diptera._ The majority of this order may be set down as _Vermiform_, though it is not improbable that some of them bear an analogy to animals that appear far removed from the _Annulosa_. Thus, the larva of _Stratyomis Chameleon_ seems to exhibit no small resemblance to some of the _Polypi vaginati_ in the _Acrita_ subkingdom of Mr. W. MacLeay[415]. That of _Culex_ and some others is constructed on a quite different type from the rest, and seems to possess some analogy to the _Branchiopod Crustacea_.

Though some of these analogies are more striking than others, yet in almost all that I have stated there is that kind of resemblance that could not be the result of what is called mere chance; and Mr. MacLeay, by first pointing out this plan of the All-wise Creator, and by laying down the doctrine of _analogies_ in general, as distinguished from _affinities_ in the animal kingdom, has furnished the believer with a new argument against those attacks of the infidel, that would render null those proofs of the wisdom and goodness of the Author of nature with which the animal and vegetable creation furnish us; by affirming most absurdly, and under the most stultifying blindness of mind, that the _creatures_ were in a manner their own _creators_, their wants under local circumstances stimulating them to efforts that in a long course of years produced all the different forms and organizations that are now to be found in our globe. The affinities and close connexion of beings with each other, so that the ascent from low to high is usually by the most gentle gradations, is the circumstance on which they build this strange and impious theory. But the fact, that certain animals of one tribe were created with a view to certain animals of another, so as to present a striking aspect of correspondence, parallel almost with that of type and antitype, without any real affinity or approximation;--this triumphantly proves a Power above and without them, who has associated them not only in a complex chain of affinities, but has caused them to represent and figure each other, even when evidently far removed, so as to give a mutual correspondence and harmony to the whole, which could be produced only by a Being infinite in power and wisdom, who made all things after a general preconceived plan and system.

iv. We are now to consider the _clothing_ with which larvæ are furnished. Many are quite naked, and smooth or rough only with granular elevations or tubercles orderly arranged; but a very considerable number, especially of the _Lepidoptera_ order, are clothed with hair or bristles of different kinds, in greater or less abundance, and arranged in different modes; and a proportion still smaller have their skin beset with spines or a mixture of spines and hairs. Lyonnet found that the hairs of the caterpillar of the great goat-moth (_Cossus ligniperda_) were hollow, though not to the apex: probably this is the case with those of other larvæ, as well as with their spines. In this instance they were set, he observes, in a corneous ring, or very short cylinder, elevated a little above the skin. The hair passes through this ring, and appears to be rooted in a soft integument, which clothes the skin within, and upon which the nerves form a reticular tissue, some of which he thinks he has even seen enter the root of the hairs, which perhaps are organs of touch[416].