54 The Form of Insects 



thorax there is no motion at all. The female cockroach 

 has vestigial wings, and the male is but a poor flyer ; 

 consequently the muscles of flight are very imperfectly 

 developed, as compared with what we And in such 

 highly-organised insects as Dragonflies, Houseflies and 

 Bees. In dragonflies each wing is provided with 

 three muscles for raising and five for depressing it ; 

 they act on the wing by means of a long lever in 

 connection with a highly complicated system of 

 sclerites (3, 16). 



Blood-system. — The course of the blood in an 

 insect differs considerably from that with which we 

 are familiar in back-boned animals. In our own 

 bodies the blood circulates entirely within a system 

 of closed vessels, but in insects there is only a single 

 vessel running from end to end of the body beneath 

 the back ; the blood is propelled forwards through 

 this and then passes into the body-cavity, completely 

 surrounding and bathing the various organs. The 

 cockroach's blood is colourless ; in some insects it is 

 coloured green, violet, or red. The blood of insects 

 contains cells which change their form, comparable to 

 the white corpuscles of our own blood. 



Heart. — In the Cockroach the greater part of the 

 single vessel forms the heart, which may be described 

 as a long tube with thin muscular walls made up of 

 thirteen chambers ; these are separated from each 

 other by valves (fig. 37). Ten of these chambers 

 correspond to the segments of the hind-body, and 

 three to those of the thorax. At a junction between 

 two chambers, the upper wall of the forward one pro- 

 jects above that of the hinder in a kind of pouch 

 {median lobe), while the side walls of the forward 

 chamber form two side pouches {lateral lobes) also 

 projecting backwards ; in the angles formed by these 

 are inlets from the surrounding space into the cavity 



