CHAP. 1. 1 DEFINITION VND STRUCTURE OF INSECTS. 13 



of tin- Nervous System, that of the alimentary tract, consists ol 

 ^ma 11 and obscure nerves in connection with the posterior j >. i rt oi 

 the digestive sj stem. 



Respiratory System. 



[nsects are .is .1 rule air-breathers and take in air through 

 special holes (spiracles or stigmata) placed along th ' the 



abdomen and thorax, thus differing widely from the higher animals 

 which breathe through the head. The thoracic spiracles are 

 usually reduced to a single pair situated in the prothorax but 

 sometimes (as in Pulicidae) all three pairs may be present. 1 

 spiracles, which can generally be opened and closed at the will 

 ol the insect, are defended against the entrance of foreign bodies 

 b> various devices, of which we may mention here the hairs found 

 on the body of many Lepidoptera and Diptera and the waxy 

 excretion produced by many Scale-insects. The spiracles com- 

 municate with .1 chamber opening into a system of tubes 'tracheae) 

 which lead the air into contact with the chyle. In some insects. 

 especiallj those living in a wel or moist environment, there are 

 often peculiar structural modifications to enable air to be obtained ; 

 the larva of the dipterous fly Eristalis, which is commonly found 

 in small accumulations of water in tree-trunks, etc., has a long 

 extensile tube which can be thrust up to the surface of the water. 

 Many aquatic insects, however, obtain their air supply direct 1\ 

 from the water by means of pseudo-branchiae, which are usually 

 flat plate-like out-growths, often fringed with hairs, occurring 

 along the sides of the body ; the Ephemerids (Mayflies) are familiar 

 examples and almost any paddy-field will yield specimen 

 Nymphula depunctalis (see Plate XXXII) in which the pseudo- 

 branchiae are seen as a series of delicate filaments all down the 

 body. It is because this caterpillar breathes in this way that it i-. 

 controlled by draining the water off the rice fields. 



( 'irculatory System. 



Insects do not possess a closed Circulatory System as found in 

 the higher animals in which the blood is circulated in special ves- 

 sels (veins and arteries), but insects have a single vessel, pulsating 

 in parts, through which the circulating liquid flows and runs 

 through the spai lae) of all the organs, being regulated in 



its course by special diaphragms which, in the shape of movable 

 connective membranes, divide these lacunas into chambers vari- 

 ously disposed. So that the circulation is partly vascular and 

 partly lacunar. The dorsal pulsating vessel (Figure 2. I'.O. also 



