982 



INSECTA. 



of perfect rest, previously to changing its skin, 

 the number is pretty nearly equal at each pe- 

 riod, being about thirty. When the insect 

 has passed into the pupa state it sinks down to 

 twenty-two, and subsequently to ten or twelve, 

 and after that, during the period of hybernation, 

 it almost entirely ceases. But when the same 

 insect which we had watched from its earliest 

 condition was developed into the perfect state 

 in May of the following spring, the number of 

 pulsations, after the insect had been for some 

 time excited in flight around the room, amounted 

 to from one hundred and ten to one hundred 

 and thirty-nine; and when the same insect was 

 in a state of repose, to from forty-one to fifty. 

 When, however, the great business of life, the 

 continuation of the species, has been accom- 

 plished, or when the insect is exhausted, and 

 perishing through want of food or other causes, 

 the number of pulsations gradually diminishes, 

 until the motions of the heart are almost im- 

 perceptible. Insects, then, do not deviate from 

 other animals, as has been supposed, in re- 

 gard to their vital phenomena, although it has 

 been somewhat curiously imagined that the 

 nutrient and circulatory functions are less 

 active in the perfect than in the larva condition. 

 This supposed inferiority has been attempted 

 to be accounted for on the hypothesis that as 

 insects no longer increase in size after entering 

 the perfect state, there is but little expendi- 

 ture or waste of body, and that, consequently, 

 they must require less nourishment. But we 

 have elsewhere shown * that the expenditure of 

 the body, whether in the larva or perfect state, 

 is in the ratio of the amount of activity and 

 length of life of the insect, while it will be 

 remembered that those insects which exist but 

 for a short time in the perfect state, and take 

 little or no food, invariably have a supply 

 of nourishment stored up within their own 

 bodies, in immense accumulations of adipose 

 matter; and that those which exist for a long 

 period have within themselves only a small 

 quantity of nourishment, but are by no means 

 sparing in the quantity of food daily consumed 

 by them, being, as they often are, some of the 

 most voracious of the insect race. 



Organs of respiration. All perfect insects, 

 whether inhabitants of air or water, breathe air 

 alone; but some larvae, that are constant in- 

 habitants of water, respire the air which is me- 

 chanically mixed with the water, by means of 

 branchiae ; but respiratory organs in the form of 

 tracheae (Jig. 435) are almost as extensively 

 distributed throughout every part of their bodies 

 as in the perfect insects. We shall divide the 

 respiratory organs into external and internal. 

 The external are of three kinds, spiracles, tra- 

 chea, and branchia. The internal are either 

 simply trucheal, or tracheal and vesicular. 



The spiracles are apertures situated along the 

 sides of the body communicating directly with 

 the internal respiratory organs. They are 

 usually nine in number on each side. In Hy- 

 menopterous larvae there are ten. Each spi- 

 racle consists of a horny ring, generally of an 

 oval form, within which is a valve formed of a 



* Phil. Trans, p. 2, 1837. 



Fig. 435. 



Portion of a tracheal vessel of the larva of Vanessa 

 urtices, shewing, a, the spiral fibre ; and 6, the 

 loose investing covering. {Newport, Phil. Trans.) 



series of converging fibres, and which opens 

 perpendicularly in its long axis, guarding the 

 external entrance. At a little distance within 

 this valve the spiracle is somewhat enlarged, 

 and there is a second valve of a more com- 

 plicated form. This has already been noticed 

 in our account of the muscular structure, but 

 we must again describe it in connexion with the 

 respiratory organs. The anterior half of this 

 inner or second valve is strong, immovable, 

 and of a horny texture, of the colour of tortoise- 

 shell. It is thin and lunated at its margin. 

 The posterior half is thick, rounded, and freely 

 moveable, and closes on the anterior like a 

 cushion or pad. This is the structure of the 

 spiracle in the Sphinx and most other insects. 

 But in some, in which the spiracle is concealed 

 beneath a portion of the skeleton, the horny 

 external ring is absent, and instead of it the 

 entrance, or margin of the spiracle, is merely a 

 little thickened and fringed with short hairs. 

 This description of spiracle exists in the pro- 

 thorax of some Coleoptera and Orthoptera 

 as in Gryllotalpa, in which one portion or lip 

 overlaps the other, thus forming as it were an 

 outer valve or lid. In other instances, as in the 

 Lamellicornes, the spiracles of the abdomen 

 are very minute and circular, and their open- 

 ing appears to be cribriform, or at most only 

 very minute, and surrounded by short hairs. 

 In others, again, as in some larvae, the spiracles 

 consist of a broad margin with a narrow 

 middle space and central aperture that leads 

 immediately into the tracheal vessel, the open- 

 ing into which is exceedingly small. The size 

 of the spiracles in different parts of the body 

 varies very much in different insects. Those 

 of the abdomen are always much smaller than 

 those of the thorax, and the most posterior ones, 

 which were of great importance in the larva 



