244 



INVERTEBRATE ZOOLOGY 



The inner pair of gonopophyses of the males are modified as an 

 intromittent organ or cirrus, while the other two pairs of gono- 

 pophyses are frequently modified as clasping organs which func- 

 tion in copulation. Through many groups the external geni- 

 talia show remarkable constancy of form within each species. In 

 many instances, the forms grouped together as single species 

 by early writers are at present being separated into several 

 clearly distinct species chiefly on the basis of characters furn- 

 ished by a study of the male genitalia. The last (eleventh) 

 abdominal somite in many insects bears a pair of caudal append- 

 ages known as cerci. Both in form and in function these are 

 highly variable. 



9 10 t1 



89 10 i\ 



Fig. 121. — Extremity of abdomen of a grasshopper, Mdanoplus dlffere?itialis; 

 A, male; B, female. The terga and sterna are numbered, c, cercus; d, dorsal 

 valves of ovipositor; c, egg guide; p, podical plate; s, spiracle, s.p., suranal plate; 

 V, ventral valves of ovipositor. (From Folsom's Entomology) . 



The Respiratory System. — The internal organization of insects 

 is essentially like that of the other arthropods already described. 

 The respiratory system offers one of the most conspicuous points 

 of difference from most other arthropodan groups. This is a 

 system of air tubes, called tracheae and tracheoles, which carries 

 air to all parts of the body (Fig. 122). This system is in com- 

 munication with the outside air through openings in the body 

 wall, termed spiracles, of which there are normally ten pairs, 

 arranged, as a rule, two pairs on the thorax and eight on the 

 abdomen. Opening and closing of the spiracles for the admission 

 or expulsion of air is under control of the insect. From each 

 spiracle a short trachea commonly leads to a main tracheal trunk 

 of which there is one on each side of the body. Branches from 

 these two main trunks penetrate into even the minutest parts 

 of the body. 



Tracheae arise as invaginations of the body wall, and con- 

 sequently the infolded chitinous covering of the body continues 



