474 THE INVERTEBRATA 



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 diff^erence 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 sub- 

 mentum 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, back-swimmer) 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 and trypanosomiasis among mammals. 



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. 328). At its base the groove does not exist but the lab rum 

 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 maxillae which are withdrawn at their base into divergent pockets in 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. 328). At rest the rostrum is bent 

 beneath the body, and when the insect feeds it is extended forward 

 and the stylets projected to penetrate the host tissues (Fig. 328). In 

 some plant-feeding species the stylets are immensely long and very 

 slender and it is difficult to explain the mechanism by which they 

 are forced into the tissues as far as the vascular bundles, but the 

 mechanical insertion of the stylets is greatly assisted by a solvent 

 action of the saliva which appears to loosen the plant cells from one 

 another and to allow the stylets to pass between. In Aphis rumicis 



