EXOPTERYGOTA 417 



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 

 filaments — 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 

 part of the nymph. The nymphs are, however, on the whole slow- 

 moving creatures, lurking well camouflaged among water weeds 

 while in wait for their prey. The main difference between the mouth 

 parts of the nymph and imago concerns the labium. In the adult this 

 has normal proportions, but in the nymph the mentum and submentum 

 are elongated and capable of being shot out rapidly from the folded 

 resting position, so impaling the prey, e.g. a tadpole, on the labial 

 hooks. 



Order HEMIPTERA or RHYNCHOTA (Bugs) 



Mouth parts for piercing and sucking ; palps absent ; labium forming 

 an incomplete jointed tube which receives dorsally two pairs of 

 slender stylets (maxillae and mandibles) ; wings usually two pairs, the 

 anterior harder than the posterior; metamorphosis gradual. 



The existence of this large order of insects has largely been de- 

 pendent on the store of easily obtainable food which exists in the sap 

 of flowering plants and the mouth parts form an efficient mechanism 

 for obtaining this. There are, however, families like the Reduviidae 

 and Cimicidae (bed bugs) and the various water bugs (e.g. Nepa, 

 water scorpion, and Notonecta, water boatman) which feed on animal 

 juices. On either count they are of immense economic importance, 

 not only for the damage which the loss of sap and blood causes to the 

 host organism, but also because they open the way for bacterial in- 

 fection and carry the agent of such diseases as "mosaic disease" 

 among cultivated plants. 



The antennae are usually short. The labium projects from the head 

 as a rostrum which is jointed, and dorsally grooved to carry the 

 stylets (Fig. 309). At its base the groove does not exist but the 

 labrum roofs over an enclosed space. The stylets are modified 

 mandibles and maxillae which are withdrawn at their base into 

 divergent pockets in the head, but converge and interlock as they pass 

 into the space between the labrum and labium and into the groove of 

 the latter, in which they fit tightly; where the inner pair of stylets" 

 (the maxillae) meet together there are left two narrow channels, of 

 which the dorsal serves for the inward passage of the food juices and 

 the ventral for the outward flow of the saliva (Fig. 309). At rest 

 BI 27 



