500 THE INVERTEBRATA 



ocelli and the absence of crotchets or spines on the abdominal legs. 

 The prolegs of the abdomen occur on different segments in the two 

 forms under consideration as reference to Fig. 344 clearly shows. 



The order, falls naturally into two suborders, the Symphyta and the 

 Apocrita. 



Suborder I, the Symphyta, includes those species with the most 

 generalised form, both as adults and as larvae. None of them show 

 the highly specialised habits and instincts which characterize most 

 of the remaining suborder, and with few exceptions they are phyto- 

 phagous. The first abdominal segment is not perfectly fused to the 

 metathorax nor is the fusion accompanied by the constricted waist so 

 characteristic of the remaining Hymenoptera (Fig. 346 D). The ovi- 

 positor is used in oviposition as a saw or drill for piercing plant tissues. 

 The trochanter is two-jointed. Larvae are cruciform (Fig. 344 D) and 

 in addition to thoracic legs, certain of the abdominal segments often 

 carry prolegs devoid of distal spines or crotchets. 



To this group belong the wood-wasps, the ovipositors of which are 

 used as a drill for perforating growing timber, in which the eggs are 

 laid. The six-legged, strong-headed larva bores through the wood (in 

 the case of Sirex gigas, this stage lasts as long as two years), pupation 

 occurring near the surface of the affected timber, from which the 

 adult bites its way out. The sawflies (Fig. 346 D), with saw-like ovi- 

 positors, are most important as agricultural pests, and are distinguished 

 from the wood-wasps by their softer bodies, their smaller size, and 

 by the presence of two apical spurs on the anterior tibiae, e.g. 

 Nematus ribesii the gooseberry sawfly. 



The second suborder, the Apocrita, includes all the remaining 

 Hymenoptera. The second abdominal segment is invariably con- 

 stricted to form a narrow waist or petiole, the first segment being 

 firmly amalgamated with the thorax (Fig. 346). Larvae are apodous 

 when full grown. 



Ichneumon flies (Fig. 346 A) are distinguished by their slender 

 curved antennae, and by the stigma on the wing. The ovipositor is 

 long and issues far forwards beneath the abdomen. The larvae of 

 Lepidoptera and of sawflies are their commonest hosts. Rhyssa 

 parasitizes the larvae of Sirex. 



Cynipid flies have similarly slender antennae, but by the absence 

 of the stigma on the wing, and by their reduced venation are easily 

 distinguished from the foregoing. Many of these are plant gall- 

 formers, e.g. Neuroterus responsible for oak galls, and Rhodites for 

 the pin-cushion galls of roses. Others, e.g. Eucoila, are parasitic on 

 fly larvae. 



Chalcid wasps (Fig. 346 B) also have a venation of the wing which is 

 so reduced as to present no closed cells. The antennae are geniculate 



