INTRODUCTION. 



17 



above the water, certain insects employ their elytra * as a sort of 

 reservoir; others make use of their antennae, the hairs of which 

 retain the globules of air. In this case 

 it is brought under the thorax, whence a 

 groove carries it to the spiracles. Some- 

 times the same result is obtained by 

 a more complicated arrangement, con- 

 sisting of respiratory tubes which can be 

 thrust into the air, which it is their func- 

 tion to introduce into the organisation. 



Insects which breathe in the water 

 without rising to the surface are provided 

 with gills — organs which, though variable 

 in form, generally consist of foliaceous 

 or fringed expansions, in the midst of 

 which the tracheae ramify in considerable 

 numbers. These vessels are filled with 

 air, but it does not disseminate itself in 

 them directly, and it is only through the 

 walls of these tubes that the contained 

 gas is exchanged for the air held in sus- 

 pension by the surrounding water. The 

 oxygen contained in the water passes 

 through certain very permeable mem- 

 branes of the gill, and penetrates the 

 tracheae, which discharge, in exchange, 

 ■carbonic acid, which is the gaseous pro- 

 duct of respiration. 



Fig. 14 represents the gills or breathing 

 apparatus in an aquatic insect. We take 

 as an example Ephe7ne7'a.\ It may be 

 observed that the gills or foliaceous 

 laminae are placed at the circumference 

 of the body, and at its smallest parts. Fig. 14. 



We have now seen that the respiratory Branchix, °>;|^||;;^f;^yq"=^tic larva 

 apparatus is considerably developed in a, foliaceous laminae, or giiis. 

 insects ; it is, therefore, easy to foresee 

 that those functions are most actively employed by them. In fact, 



* The horny upper wings with which some insects are provided are called 

 elytra.— Ed. 



t May-fly family. — Ed. 



C 



