I N S 



variations afford grounds for classifications of the 

 larvae, but which it would be tedious to detail without 

 entering more fully into minute particulars than our 

 space will allow. In like manner the various horns, 

 spines,and protuberances must be passed over in silence, 

 especially as we have already in our article CATER- 

 PILLAR, and upon various other occasions, noticed any 

 remarkable variations which may occur in those res- 

 pects. It remains, however, to be noticed, that the 

 terminal segment of the body is often very different 

 from the preceding in its form, and that its underside 

 in some hexapod larva is furnished with a retractile 

 tubercle, which the insect sometimes employs as a 

 seventh leg. 



In addition to the organs above described, are to 

 be noticed the spirncles, or apertures by which the 

 larvae take in a supply of air. These spiracles are 

 not, as in the higher animals, situated in the head, 

 but are arranged in a series on each side of the body, 

 opening into two longitudinal tubular internal vessels 

 called trachea, from which an immense number of 

 ramifications diverge to every part of the body of the 

 insect. These spiracles are placed in the first, fourth, 

 and following segments (except the last), and consist 

 of a small elongated opening surrounded by a callous 

 margin (fig. 25 represents a spiracle open', and 26 a 

 closed spiracle). In aquatic larvae, however, a variation 

 in the organization of the animal necessarily takes 

 place, as it is not exposed to the ordinary action of the 

 air. We have seen already in our article DYTICID.E 

 and DRAGON-FLY, how this iseffectedin these insects; 

 but in other aquatic larvae, as in the EPHEMERAE, the 

 sides of the body are furnished with elongated flattened 

 plates, through which a slender air tube meanders, 

 and which communicates with the longitudinal air 

 tubes above-mentioned, (fig. 32.) Other variations 

 occur in the larvse of the gnats (Culicidec) and midges 

 (Ckironomus), which we have already described. But 

 the most curious variation which occurs in this respect, 

 is found in the very common larvae of the Hdopkieus 

 pendulus, which has been termed the rat-tailed grub, 

 from the peculiar formation of the extremity of the 

 body, which is very slender and elongated, enclosing 

 a second still more slender air tube, which is capable 

 of being protruded, so as to be pushed to twelve times 

 the entire length of the body. As the insect lives in 

 mud. this structure is eminently serviceable in enabling 

 it to obtain a due supply of air. 



There are, however, some considerations resulting 

 from the variations in the form of the larva? of insects 

 which ought not to be passed over without notice, in- 

 asmuch as the subject is one of great interest, hitherto 

 but little cultivated, and one of much importance as 

 regards the classification of the annulose sub-kingdom. 

 We have already observed that the immature state of 

 insects typified the perfect forms of those particular 

 grades through which the winged type may be sup- 

 posed to have passed in its progress to maturity. 

 Without arriving at this theory as stated above, Mr. 

 MacLea>,in his Horae Entomologies, and Messrs. 

 Kirby and Spence, have given a series of analogies 

 exhibited by the larvae of insects with other annulose 

 groups; the former contending that it is only by the 

 assistance of such analogies that the real mode of dis- 

 tribution (by which every variation shall have its due 

 weight appropriated to it), and the consequent dis- 

 covery of the natural system, can be attained ; thus the 

 larva? of the cockchafer, and other lamellicorn 

 beetles, represent a full-grown lulus in its peculiar 



E C T 837 



I mode of rolling itself into a coil on one side ; and 

 the same analogy is exhibited by the larvce of the 

 sawflies. It would lead us too far to enter more at 

 length into these views which, however, it wou.u 

 have been improper to have entirely overlooked. 



We have said that it is during ti.e ia-va state tha 

 the chief supply of nutriment is taken by the larva 

 It will be proper, therefore, that we should enter into 

 a few particulars relative to the voracity of insects at 

 this period of their existence, and which, in propor- 

 tion to their size, far exceeds that which is exhioited 

 by any of the larger animals. Thus Redi discovered 

 that the larva? of the flesh fly had become, in the 

 space of twenty-four hours, at least two hundred 

 times heavier than before. And the Count Daudolo 

 gives the following as the result of the most exact 

 calculations made from the observations of the culti- 

 vators of silk, who know the exact weight of thtf 

 leaves devoured by the caterpillars ; 1609^ pounds 

 weight of leaves being consumed by the progeny 

 raised from an ounce weight of eggs. 



First age, 6 Ibs. of sorted leaves, 1 Jibs, refuse 



Second age, 18 ,, 3 



Third age, 60 9 



Fourth age, 180 27 



Fifth age, 1098 1112 



105 allowed for evaporation 



1362 



Deduct fur- 

 ther allow- 

 ance for lit- issi 

 ter, uneaten 

 leaves, &c. 



I206ilbs. actually devoured. 



But it is to be observed, however, that the stomachs 

 of these insects, like that of the horse (see HORSE), 

 does not possess the power of dissolving these leaves 

 in the most perfect manner, but only of extracting a 

 juice from them. Indeed this very circumstance is 

 assigned by John Hunter (Observations on the Ani- 

 mal Economy, page 221, quoted by Kirby and Spence) 

 as the probable proximate cause for the voracity of 

 herbivorous larvae. And hence, of the 1206^- pounds 

 of leaves actually devoured, 745 pounds are depo- 

 sited as excrement in an indigested state. Hence it 

 is evident that in comparison with the stomach of the 

 perfect insect, in which state but very little food is in 

 general taken, and in some cases the insect is even 

 totally destitute of a mouth, the stomach of the 

 caterpillar, and its apparatus for taking its food, must 

 be fully developed ; and this is found to be the case, 

 the stomach occupying a considerable portion of its 

 interior, and the organs of the mouth being very 

 robust. The caterpillar of the goat moth is three 

 years in arriving at its full size, when it is 72,000 

 times heavier than when newly hatched ; and a silk- 

 worm, weighing when first hatched 1-1 00th part of a 

 grain, consumes in thirty days about 60,000 times its 

 primitive weight. 



These particulars may suffice for the voracious 

 powers of individual insects. It is when they have 

 been produced in considerable numbers together that 

 these powers are rendered more widely perceptible ; 

 and in fact become highly prejudicial, as we have al- 

 ready endeavoured to shew in the early part of this 

 ARTICLE. An instance or two may not, however, be 

 here out of place. Of these, the first we shall men- 

 tion is the salt-marsh caterpillar of North America, 

 of which an interesting account has been published 



