472 THE INVERTEBRATA 



or leaves and covered by a protecting sheath of silk by the female, 

 e.g. Peripsocus phaepterus. 



Atropus pulsatoria, the booklouse, is found in damp dark rooms 

 and feeds on the paste of book bindings, wallpaper, etc. 



Order O DON AT A (Dragonflies) 



Predaceous insects with biting mouth parts; two similar pairs of 

 wings with characteristic reticulate venation; prominent eyes and 

 small antennae ; elongated abdomen with accessory male genitalia on 

 the 2nd and 3rd sterna; metamorphosis heterometabolous ; nymphs 

 aquatic, possessing a modified labium known as the mask. 



The members of this order are large insects, and in the Carboni- 

 ferous period genera existed which had a wing expanse of two feet. 

 They are strong and rapid fliers, catching their food in the form of 

 small insects, on the wing. The forwardly directed legs play an im- 

 portant part in catching the prey and holding it w^hile it is masticated 

 by the mouth parts. 



The thorax has a peculiar obliquity of form, the pleural sclerites 

 being directed downwards and forwards at each side with the result 

 that the leg bases are carried forwards towards the mouth and the 

 wing bases backwards. 



The wings (Fig. 327) have a complex venation of a reticular nature, 

 characteristic features being a stigma or chitinous thickening of the 

 wing membrane near the apex, a nodus or prominent cross vein at 

 right angles to the first two longitudinal veins, and a complex of 

 veins near the wing base known as the triangle, Fig. 327. There is no 

 coupling apparatus. All the mouth appendages are strongly toothed, 

 maxillae and labium assisting the mandibles more efficiently in 

 mastication than in most insects with biting mouth parts. 



Though the male pore is on segment 9 of the abdomen, the copu- 

 latory apparatus is found in the sternal region of segments 2 and 3. 

 Before copulation, spermatozoa are transferred to this apparatus. 

 The male then grasps the female in the region of the prothorax by 

 means of his posterior abdominal claspers. While in flight in this 

 tandem position the female turns her abdomen down and forwards 

 and receives sperm from the accessory copulatory apparatus of the 

 male. Dragonfly eggs are laid in water or on water weeds. The 

 nymphs breathe by means of tracheal gills and are of two kinds: 

 (i) those with external gills in the positions of cerci anales and caudal 

 fWaments—Zygoptera, (ii) those with gills on the walls of the rectum 

 — Anisoptera. In the latter case water is pumped in and out through 

 the anus, and this action may be made use of in locomotion — the 

 sudden expulsion of water causing a rapid forward movement on the 



