1 8 ECONOMIC ENTOMOLOGY 



the ventral, inner and dorsal plates. The inner pair usually forms the 

 ovipositor or the intromittent organ, and the outer pairs may form a 

 sheath or claspers. 



The Genitalia. — A knowledge of the genitalia is of importance to 

 the taxonomist because in certain families the separation of species 

 is based largely on these structures. It is also of importance to the 

 economic entomologist because it enables him to understand the 

 method of egg deposition in injurious and beneficial insects and 

 the poisonous action in certain forms in which the ovipositor has been 

 modified to form a sting. 



Fig. 23. — Abdomen of female house-fly, showing extended ovipositor, a., Anus; c, 

 cercus; d.p., dorsal plates; v. p., ventral plate. 



There is no true ovipositor in the Neuropteroida, Coleoptera, 

 Lepidoptera or Diptera, the vagina opening directly to the exterior 

 (Fig. 23). In some of these insects, however, a whip-Hke or tubular 

 pseudo-ovipositor is formed by the last few segments of the abdomen 

 {e.g.y Cerambyx, Cecidomyia, Musca). 



A true ovipositor is developed in the Thysanura, Orthopteroida, 

 Hemiptera and Hymenbptera (Fig. 24). In the Orthoptera the gona- 

 pophyses are used for making holes in the ground or shts in stems for 

 the reception of the eggs. In the Hymenoptera there are various 

 peculiar modifications of the ovipositor: Megarhyssa, one of the larger 

 ichneumon flies, uses its long ovipositor as a drill, forcing it, in spite 

 of its extreme slenderness, up to the hilt in the trunk of hardwood 

 trees in order to deposit its eggs in the burrows of the horn-tail borer 



