Part 12

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.