26 



ELEMENTARY ENTOMOLOGY 



ovipositors, with which they are enabled to insert their eggs in the 

 ground or in wood, but the greatest development of the ovipositor 

 is found among hymenopterous insects in which it is formed for 

 sawing, boring, or stinging. Another pair of jointed appendages, 



c 



c> 



FIG. 29. Ovipositor of periodical cicada 

 At rest at A, and exserted at B 



called cerci, are frequently found attached to the tenth abdominal 

 segment. They are quite variable in length, but in May-flies are 

 as long as the body and resemble very slender antennae projecting 

 backward from the abdomen. In most cases they are tactile organs, 



but in the cockroach they assist in 

 smelling. 



The number of visible abdom- 

 inal segments varies from five to 

 eleven in different orders, and fre- 

 quently the number is different on 

 the upper, or dorsal, and under, or 

 ventral, sides. The structure of the 

 anal segments is usually different 

 in the sexes and furnishes impor- 

 tant characters for classification. 



Integument. Before studying 

 the internal anatomy, the skin, or 

 integument, of the insect should 

 be considered. This has become 



hardened so that it forms a firm outer skeleton, to which the mus- 

 cles and internal organs are attached. Thus the parts of the insect 

 skin, as have been described, are analogous to the bony skeleton 



-- h 



FIG. 30. Section through skin of a 

 beetle (Chrysobothris] 



l>, basement membrane ; c 1 , primary cutic- 



ula ; c 2 , secondary cuticula ; //, hypoder- 



mis cell ; , nucleus. (After Tower, from 



Folsom) 



