880 



INSECTA. 



by the cutaneous and respiratory surfaces. But 

 when the changes in the internal structures are 

 nearly completed, and the perfect insect is soon 

 to be developed, the respiration of the pupa is 

 greatly increased, and the gaseous expenditure 

 of its body is augmented in the ratio of the 

 volume of its respiration, which is greatest the 

 nearer the period of development. Thus in the 

 same insect in which the diminution of weight 

 was so trifling during eight months' quiescence 

 and abstinence, it amounted in the succeeding 

 fifty-one days to nearly half the original weight 

 of the pupa, since the perfect insect, imme- 

 diately after its appearance on the 24th of May, 

 weighed only thirty-six grains. 



This increased activity of function is attended 

 with a correspondent alteration in the general 

 appearance of the pupa. In the sphinx all the 

 parts of the future Imago become more and 

 more apparent on the exterior of the pupa case, 

 the divisions into head, thorax, and abdomen 

 are more distinctly marked, the eyes, the an- 

 tennae, and the limbs appear as if swollen and 

 ready to burst their envelope, and the pupa 

 gives signs of increasing activity by frequent 

 and vigorous contortions of its abdominal seg- 

 ments. The naked pupa or nymph, in which, 

 as we have seen, all the parts of the body are 

 free, and encased only in a very delicate mem- 

 brane, acquires a darker colouring and a firmer 

 texture, while the species which undergo their 

 metamorphoses into nymphs in the water, Tri- 

 choptera, the caddis-flies, acquire a power of lo- 

 comotion as the period of their full develop- 

 ment approaches, to enable them to creep up 

 the stems of plants, and leave that medium in 

 which it is impossible for them to exist as per- 

 fect insects. 



In every instance the assumption of the per- 

 fect state is accompanied by a slipping off of 

 the external covering. Before this can be ef- 

 fected, many Lepidoptera, like the Trichoptera, 

 have first to remove themselves from the locality 

 in which they have undergone their previous 

 metamorphoses. When this happens to be in 

 the interior of the trunks of trees, or in other 

 situations from which it is difficult to escape, 

 the abdominal segments of the pupa are often 

 beset with minute hooks (fig. 367), similar to 

 those on the feet of the larva. By means of 

 these, by alternately contracting and extending 

 its abdominal segments, the pupa is enabled to 

 force an opening through its silken coccoon, or 

 to move itself along until it has overcome the 

 obstacles which might oppose its escape as a 

 perfect insect. 



The imago or perfect state. Immediately 

 after the insect has burst from the pupa case it 

 suspends itself in a vertical position with its 

 new organs, the wings, somewhat depending, 

 and makes several powerful respiratory efforts. 

 At each respiration the wings become more and 

 more enlarged by the expansion and extension 

 of the tracheal vessels within them, accompa- 

 nied by the circulatory fluids. When these 

 organs have acquired their full development 

 the insect remains at rest for a few hours and 

 gains strength, and the exterior of the body be- 

 comes hardened and consolidated, and forms, 



what we shall presently consider, the Dermo- 

 skeleton. This is what takes place in Lepidop- 

 terous insects. Some of the Coleoptera, as in 

 the instance of Melolonthu vulgaris, the com- 

 mon chaffer-beetle, remain for a greater length 

 of time in their nidi before they come abroad 

 after entering the imago state. This is also the 

 case with the Humble-bees. When these in- 

 sects first come from their cells they are exceed- 

 ingly feeble, their bodies are soft, and covered 

 with moisture, their thick coating of hairs has 

 not acquired its proper colour, but is of a gray- 

 ish white, and they are exceedingly susceptible 

 of diminished warmth. They crowd every 

 where among the cells, and among other bees, 

 where there is most warmth. In a few hours 

 this great susceptibility is diminished, and their 

 bodies acquire their proper colours, but they 

 do not become sufficiently strong to be capable 

 of great muscular exertion, and undertake the 

 labours of the nest until the following day. 



When an insect has once entered its perfect 

 state, it is believed to undergo no further meta- 

 morphosis or change of covering. But there 

 exists an apparent exception to this general law 

 in the Ephemeridtf, which are noted for the 

 shortness of their existence in the imago state. 

 When these insects have crept out of the water, 

 and rid themselves of the pupa covering, and 

 their wings have become expanded, they soon 

 take flight, but their first movements in the air 

 are performed with some difficulty, and they 

 shortly alight again and throw off a very deli- 

 cate membrane with which every part of the 

 body has been covered, and then resume their 

 flight with increased activity. The condition 

 of the insect previously to this final change has 

 been called by Mr. Curtis the pseudimago state. 

 It was noticed long ago by Swammerdam, and 

 has usually been thought to be peculiar to the 

 Ephemerida, but occurs also in the Lepidoptera 

 and Diptera* but in them takes place at the 

 same time with the change from the pupa state. 

 Swammerdam thought the change peculiar to 

 the males of the Ephemerid<z, but Mr. West- 

 wood has seen it also in the females. 



Many insects, of which the Ephemerida and 

 BombycidsE are known examples, take no food 

 in the perfect state, and exist only for a few 

 hours, or at most only a few days, the business 

 of life being almost entirely devoted to the pro- 

 pagation of the species. In every instance of 

 the entire abstinence of a species in the perfect 

 state there is a corresponding atrophy of the 

 parts of the mouth. This we shall find is. the 

 case in the Ephemera, in the gad-fly, (Estrus, 

 and in the silk-worm moth. In the latter in- 

 stance the parts of the mouth are simply so 

 much diminished in size as to be unfitted for 

 taking food; in the former they have almost 

 disappeared. On the other hand, when the life 

 of the imago is continued for a long period, all 

 the parts of the mouth are fully developed. 

 The duration of life in these species often ex- 

 tends for many weeks, or in some even months, 

 and the quantity of food taken is consequently 

 greater than is taken by the larva. In those 



* Westwood's Introduction, &c. vol. ii. p. 28. 



