IV. INSECTA: HEXAPODA 



413 



ing an irritating substance (cantharidin) through the Jointing memljranes 

 of the legs as a means of defense. 



Tlie Hexapoda are dioecious. The paired gonads consist of a few or 

 many ovarial or testicular tubules (fig. 454), in the abdomen. Their 

 paired ducts (oviducts, vasa deferentia) open separately in the Ephemerida 

 and young Apterygota, but all other Hexapoda have a single ventral un- 

 paired sexual opening just in front of the anus. This arises as a median 

 invagination of the ectoderm (hence lined with 

 chitin), which extends inwards and meets the 

 genital ducts (modified nephridia). The re- 

 ceptaculum seminis, a sac connected with the 

 female genitalia, has a special biological in- 

 terest. In insects which copulate but once 

 during life it retains the spermatozoa for a 

 long time (four years in bees) in a living con- 

 dition. As the eggs are laid they may be 

 impregnated by spermatozoa from it. Since 

 a firm shell or diorion is developed around the 

 egg in the ovary, entrance of spermatozoa is 

 only possible by a micropylar apparatus, a 

 system of tubes penetrating the chorion at one 

 end of the egg. 



Fig. 



4^S-- 



-Ventral view 



sac; d, poison giand; g, its 

 duct; ga, teriTiinal ganglion, 

 beside it accessory gland; 

 I, stylet; 2, groove of sting 

 (black); 3, sheath of sting; I, 

 angle piece; II, quadrate 

 plate for attachment of 

 muscles. 



Oviposition occurs in many insects by means of °f ^''"S of bee (after Packard 

 an ovipositor which may be developed in two ways. ■ ^"'^ Cheshire). _ W,_ poison 

 In beetles, flies and butterflies the last somites of 

 the body are small, and are normally retracted into 

 the body but can be protruded as a long tube for 

 oviposition. In Hymenoptera, Hemiptcra, Orthop- 

 tera and dragonflies the ovipositor (tercbra) is 

 formed by special appendages, the gonapophyses, 

 four to six in number, which arise from the ventral 

 side of the eighth and ninth abdominal segments. 



In the Orthoptera two pairs of gonapophyses of the eighth and ninth somites 

 form a sheath in which two other parts, the egg-guide, also derived from the 

 ninth, are enclosed. In the Hymenoptera (fig. 455) the latter are fused to a 

 tube in which both parts of the eighth somite play as a piercing stylet, while 

 two other parts lie at the sides as the sheath. In the wasps and bees these parts 

 can be withdrawn into the body, and have frequently been converted into a 

 sting (aculeus) provided with a poison f;land, which is confined to the female. 

 In the males there is usually a protrusible penis which is frequently composed 

 of the same parts as the ovipositor; in others of metamorphosed somites. 

 Further sexual differences lie in the form of the antenna;, shape and color of 

 the wings, modifications of the eyes, etc. (fig. 74). 



In many insects the eggs may develop parthenogenetically. Plant lice and 

 scale insects reproduce for generations asexually, and parthenogenesis is widely 

 distributed among Hymenoptera, Lepidoptera, and Neuroptera. Tlie condi- 



