INSECTA. 



875 



average weight is about ninety-five grains, and 

 its length sometimes as much as forty lines. 

 During this period, therefore, it has increased 

 nine thousand and five hundred times its origi- 

 nal weight, and has eaten sixty thousand times 

 its weight of food. But observations on the 

 larva of the privet hawk moth, Sphinx lignstri,* 

 lead us to believe that this estimate of the 

 amount of food eaten is a little too great. The 

 larva of the sphinx at the moment of leaving 

 the egg weighs about one eightieth of a grain ; 

 at about the ninth day it casts its second skin 

 and then weighs about one-eighth of a grain : 

 on the twelfth day it changes its skin again 

 and then weighs rather more than nine-tenths 

 of a grain. On the sixteenth day it casts its 

 fourth skin and weighs three grains and a half, 

 and on the twenty-second day enters its sixth 

 and last skin and weighs very nearly twenty 

 grains; but on the thirty-second day, when it 

 has acquired its greatest size, it weighs nearly 

 one hundred and twenty-five grains, so that in 

 the course of thirty-two days this larva increases 

 about nine thousand nine hundred and seventy- 

 six times its original weight. At this period it 

 is sometimes more than four inches in length. 

 But this is not the greatest weight that the 

 larva attains. One specimen which was bred 

 in its natural haunts weighed one hundred and 

 forty-one grains and seven-tenths, so that in 

 this instance the insect had increased at the rate 

 of eleven thousand three hundred and twelve 

 times its original weight. But great as is this 

 proportion of increase, it is exceeded by some 

 other larvae. Lyonet found that the larva of 

 Cossus ligniperda, which remains about three 

 years in that state, increased to the amount of 

 seventy-two thousand times its first weight.f 

 This amazing increase is occasioned chiefly by a 

 prodigious accumulation of fat which exists in 

 a greater quantity in this than in most other 

 larvae. We have ourselves removed forty-two 

 grains of fat from one specimen, which was 

 more than one-fourth of the whole weight of 

 the insect. The occasion for this prodigious 

 accumulation is chiefly to supply the insect 

 during its continuance in the pupa state, while 

 the muscular structure of the limbs and other 

 parts of the body are in the course of develop- 

 ment ; and also to serve, perhaps, as an imme- 

 diate source of nutriment to the insect at the 

 period of its assuming the perfect state, and 

 more particularly during the rapid development 

 of its generative functions ; since, when these 

 have become perfected, the quantity that re- 

 mains is very inconsiderable. But all larvae do 

 not increase in these amazing proportions, 

 although their actual increase may be more 

 rapid." Those in which the proportion of in- 

 crease is the greatest are usually those which 

 remain longest in the pupa state, as in the 

 species first noticed. Thus Redil observed in 

 the maggots of the common flesh-flies a rate of 

 increase amounting to about two hundred times 

 the original weight in twenty-four hours, but 

 the proportion of increase in these larvae does 



* Phil. Trans. 1837, part ii. p. 315. 

 t Traite Anat. de la Chenille, p. 11. 

 J De General. Insectorum, p. 27. 



not at all approach that of the sphinx and 

 cossus. From observations made on the larva 

 of one of the wild bees, Anthophora retusa, we 

 believe that this is also the case with the Hy- 

 menoptera. The weight of the egg of this 

 insect is about the one hundred and fiftieth 

 part of a grain, and the average weight of a 

 full-grown larva six grains and eight tenths, so 

 that its increase is about one thousand and 

 twenty times its original weight ; which, com- 

 pared with that of the sphinx of medium size, 

 is but as one to nine and three-quarters, and to 

 a sphinx of maximum size only as one to a 

 little more than eleven. 



The changes of skin which a larva undergoes 

 before it enters the pupa state are more or less 

 frequent in different species. In the generality 

 of Lepidopterous insects it occurs about five 

 times, but in one of the tiger-moths, Arctia 

 Caja, according to Messrs. Kirby and Spence,* 

 ten times. A few hours before the change is 

 to take place the larva ceases to eat and remains 

 motionless, attached by its abdominal legs to 

 the under-surface of the twig or leaf upon 

 which it has been feeding. Many species spin 

 a slight web or carpet of silk in which they 

 attach their posterior legs, as observed by Dr. 

 Pallas of Apatura iris,-\ and in this manner 

 await their change, which appears to be attended 

 with much uneasiness to the insect. The whole 

 body is wrinkled and contracted in length. In 

 the sphinx this contraction occurs to so great 

 an extent in some of the longitudinal muscles 

 of the anterior and middle part of the body 

 that the larva assumes that peculiar attitude 

 from whence the genus derives its name. In 

 this attitude the larva remains for several hours, 

 during which there are occasionally some 

 powerful contractions and twitchings of its 

 whole body, the skin becomes dry and shri- 

 velled, and is gradually separated from a new 

 but as yet very delicate one which has been 

 formed beneath it, and the three or four anterior 

 segments are greatly enlarged on their dorsal 

 but contracted on their under surface. After 

 several powerful efforts of the larva the old 

 skin cracks along the middle of the dorsal 

 surface of the second segment, and by repeated 

 efforts the fissure is extended into the first and 

 third segments, and the covering of the head 

 divides along the vertex and on each side of 

 the clypeus. The larva then gradually presses 

 itself through the opening, withdrawing first its 

 head and thoracic legs, and subsequently the 

 remainder of its body, slipping off the skin 

 from behind like the finger of a glove. This 

 process, after the skin has once been ruptured, 

 seldom lasts more than a few minutes. When 

 first changed the larva is exceedingly delicate, 

 and its head, which does not increase in size 

 until it again changes its skin, is very large in 

 proportion to the rest of its body. In a few 

 hours the insect begins again to feed most 

 voraciously, particularly after it has entered its 

 last skin, when its growth is most rapid. Thus 

 a larva of Sphinx ligustri, which at its last 



* Vol. i. 



t Trans. Ent. Society, vol. ii. part ii. p. 138. 



