INTERNAL ANATOMY OF INSECTS. 75 
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®. 
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 and 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 larvee 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®, which has an anal respiratory 
tube in its first state, thoracic respiratory horns in its se- 
cond, and the ordinary lateral spiracles in its third. 
Changes also take place in their internal organs. In 
the larvee the respiratory apparatus, especially the tra- 
cheal tubes, is often much larger and more ramified than 
in the imago; and as the former is the principal feeding 
state, there seems good ground for Mr. B. Clark’s opi- 
* De Geer i. 531—. ¢. xxvii. f. 13.5. Compare Reaum. i. 396—. 
> See above, p. 51—. 
