THE ANATOMY OF INSECTS. 41 



surface, forces out the air from beneath its wings and takes in a new 

 supply. Water-beetles and aquatic bugs afford familiar examples of 

 this mode of respiration. 



Some insects are provided with long tubes connected with their 

 spiracles by means of which they can draw their supply of air from 

 above the surface of the water while they crawl upon the bottom of 

 shallow ponds. Our most common illustrations of this are bugs of 

 the family Nepidae ; but the most remarkable development of this 

 kind is exhibited by certain Dipterous larvae of the family Syrphidae, 

 known as Rat-tailed Maggots. 



Although there are many insects that live in the water and draw 

 their supply of air from above it, the greater number of aquatic in- 

 sects breathe, as do fishes, the air that is mixed with the water. This 

 is accomplished by organs known as tracheal gills. These are hair- 

 like or more or less plate-like expansions of the body-wall, abun- 

 dantly supplied with tracheae. These tracheae divide and subdivide, 

 and their terminations or fine branches are separated from the 

 water that bathes the organ only by its thin walls. In this way the 

 air contained in the tracheae is separated from the air in the water 

 only by a delicate membrane which admits of the transfer of gases 

 between them. It will be observed that the difference between a 

 tracheal gill and a true gill (as of fishes, Crustacea, etc.) is that the 

 true gill is supplied with vessels containing blood, which is purified 

 by being brought in contact with the air in the water, while the 

 tracheal gill is supplied with tracheae containing air to be purified. 



Tracheal gills are usually borne by the abdomen, 

 sometimes" by the thorax, and in case of one genus of 

 Stone-flies by the head. They pertain almost exclusively 

 to the immature stages of insects ; but Stone-flies of the 

 genus Pteronarcys retain them throughout their existence. 



Tracheal gills vary greatly in form ; in Corydalus they 

 are hair-like and occur in tufts near the lateral margins 

 of the abdominal segments ; in the Caddice-worms they 

 are thread-like, more or less branched, and irregularly dis- 

 tributed over the surface of the abdomen ; and in certain Fig. 55 . 

 Dragon-flies (Agrioninae) they are in the form of large g m he of 

 plate-like caudal appendages, Fig. 55. 



The respiratory movements of 'insects have been carefully studied 

 by Plateau* and others. These movements consist in general of 



* See account by Plateau, in " The Cockroach," by Miall and Denny, p. 159. 



