INTERNAL ANATOMY OF INSECTS. 75 



spire air. De Geer has given an interesting record of 

 this, in the case of Hydrocampa stratiotata. This insect 

 spins a double cocoon, the outer one thin, and the inner 

 one of a close texture. In the pupa there are three pair 

 of conspicuous spiracles on the second, third, and fourth 

 segments of the abdomen, which are placed on cylindri- 

 cal tubes, and they appear to have no other air-vessels. 

 The respiratory gills of the larva having vanished, like 

 some others of the same genus, they know how to sur- 

 round themselves with an atmosphere of air in the midst 

 of the water, so that the interior of their inner cocoon is 

 impervious to the latter element how they renew the 

 air has not been ascertained. Though they respire air, 

 water is equally necessary, for the animal died when kept 

 out of water a . 



The great majority of insects respire in much the same 

 manner in all their states, particularly as to their external 

 organs; for when the larva breathes by the lateral spira- 

 cles, the pupa arid imago usually do the same. The con- 

 verse of this, however, by no means holds; for it not un- 

 frequently happens that the two latter breathe by means 

 of lateral spiracles, though they received the air in their 

 larva state by an apparatus altogether different. Thus 

 the larvae of many Diptera breathe by an anal tube, while 

 the pupa and imago follow the general system. Some- 

 times a tribe of insects breathe by an apparatus quite 

 different in all their states, as we have seen to be the case 

 with the common gnat b , which has an anal respiratory 

 tube in itsjirst state, thoracic respiratory horns in its se- 

 cond, and the ordinary lateral spiracles in its third. 



a De Geeri. 531 .f.xxxvii./. 13. s. Compare Reaum. ii. 396. 

 b See above, p. 51 . 



