XXX INTRODUCTION. 



proceed vary in size and are not invariable in position, being some- 

 times on the upperside and sometimes on the underside of the 

 head, and in some families produced into a lateral process or spine. 

 The number of joints is not a regular quantity : in the Pentatomidae 

 genera are found with antennae of either four or five joints; in 

 some Eeduviids the antennae are apparently eight-jointed, the 

 maximum number of about twenty-five being attained in the males 

 of some Coccidae. These joints vary in thickness, but are usually 

 more or less symmetrical, though in the Coreidae some genera, 

 notably Dalader and its allies, have the third joint dilated and pro- 

 minentlv compressed. The rostrum originates from the underside 

 of the head, which is excavated to receive the first rostral joint in 

 repose ; it is usually three- or four-jointed, and apically encloses 

 or enwraps four fine setae, which by some entomologists are con- 

 sidered to represent the mandibles and maxillae found in other 

 insects. This subject, however, cannot be discussed here ; but 

 Sharp, who has investigated the question, may be best quoted 

 as to the use of the organ : — " The rostrum being extended from 

 its position of repose, the tip of the sheath is brought into contact 

 with the object to be pierced, the surface of which is probably 

 examined by means of sensitive hairs at the extremity of the 

 sheath ; these therefore functionally replace to some extent the 

 palpi of other insects. As a rule, the sheath does not penetrate 

 (though there is reason for believing that in various of the animal- 

 feeding bugs it does so), but the setae are brought into action for 

 piercing the skin of the plant ; they are extremely sharp, and the 

 outer pair are usually barbed, so that when once introduced a hold is 

 easily maintained. This being established, it is thought that the 

 salivary pump comes into play, and that a fluid is injected into the 

 object pierced so as to give rise to irritation or congestion, and thus 

 keep up a supply of fluid at the point operated on ; this fluid extends 

 along the grooved setae by capillary attraction, and the rapidity of 

 the current is increased by a pumping action of the pharynx, and 

 possibly by movements of the setae themselves. Though the setae 

 are often extremely elongate — sometimes several times the length 

 of the body — they are nearly always slender, and there is no reason 

 to suppose that a perfect, or air-tight, tube is formed ; hence it is 

 probable that capillary attraction is really the chief agent in the 

 ingestion of the fluid. The slight diversity of structure of the 

 Ehynchotal trophe* is in very striking contrast with what we flnd 

 in mandibulate insects, such as Diptera and some divisions of 

 Hymenoptera. Schiodte, in commenting on this, has suggested 

 that it is probably due to the small variety of actions the rostrum 

 is put to." (Cambridge Nat. Hist. vol. vi, pp. 536-7.) 



The thorax is composed of three segments — the prothorax, 

 mesothorax, and metathorax, the upper surfaces of which are 

 known by the names of prouotum, mesonotum, and metanotum : 



* Or buccal appendages^bucculce. 



