INSECT A. 225 



upper part of each abdominal segment ; in the Semblidae plumose 

 branchiae are similarly situated. 



The amount of respiration is directly as the degree of the activity 

 of the insect ; and its temperature is increased in an approximate de- 

 gree. The extraordinary development of the breathing organs de- 

 monstrate their essential relations to the energies of the muscular 

 system ; and, by a minor modification, they are made subservient to 

 the diminution of the weight of the insect. In the Apterous insects, 

 and especially the Myriapoda, there is no trace of air vesicles, but 

 both in the Centipede and lulus the minute tracheae ramify throughout 

 the body. 



LECTURE XVIII. 



INSECTA. 



The sexes are distinct in all Insects : in certain social species, as 

 bees and ants, there is a third kind of individuals, commonly called 

 neuters, but these are essentially females with the organs imperfectly 

 developed and passive. 



In the Centipede the testes are small, slender, fusiform bodies, 

 placed one behind the other in the dorsal region of the posterior part 

 of the body : four pairs and an odd one are on the left side, and three 

 pairs on the right. A minute efferent tube is continued from both 

 ends of each testis, which tubes unite with those of the adjoining 

 organ, and ultimately form a single vas deferens, common to the two 

 lateral series of testes, and situated along the middle of the body : it 

 forms a kind of epididymis by its zig-zag convolutions, and, after 

 having received the ducts of three pairs of small prostatic glands, it 

 terminates in the cloaca.* In the lulus the external openings of the 

 male apparatus are situated upon a small protuberance behind the 

 seventh pair of legs. 



In the Ilexapod Insects the testes usually present a more compli- 

 cated structure, but are almost as diversified in the form, number, and 

 disposition of their secerning follicles as the stamens of plants. They 

 present their most simple condition in the Lepidoptera, in which they 

 approximate during the metamorphosis, and unite into a single globular 

 mass, the primitive separation of the two testes being indicated by a 



* Prof. R. Jones and Strans Dtirckheiin. 



