320 THE BIOLOGY OF INSECTS 



the foremost abdominal segment, which apparently belongs 

 to the thorax. It is worthy of note that this structural 

 character facilitates the free and accurate movement of the 

 abdomen, so that the backwardly directed ovipositor can 

 be brought to bear on any spot where the female can 

 profitably practise egg-laying. It has been mentioned 

 that there are many families of Apocrita and for distinguish- 

 ing these various structural features are of service to the 

 student. The pronotum or dorsal shield of the prothorax 

 extends backwards at the sides as far as the tegulae at the 

 bases of the forewings in most Symphyta, and this is also 

 seen (Fig. 77, a) in several large families of the Apocrita — for 

 example, in the Cynipidae (Gallflies), the Proctotrypidae (a 

 large family of small flies with parasitic larvae), the Pompi- 

 lidae (a family of digging-wasps that usually provision their 

 nests with spiders), the Vespidae or true wasps (Fig. 77, a), and 

 the Formicidae or Ants. On the other hand, the pronotum 

 is relatively short (Fig. 77, b) in the Chalcididae (flies with 

 parasitic larvae), the Sphegidae (digging-wasps that provision 

 their nests with caterpillars and other insects), and in the 

 Apidae (bees). The trochanter or small second segment of 

 the leg is divided into two regions (Fig. 77, d) by an apparent 

 joint in the Ichneumonidae, Chalcididae, and Proctotrypidae, 

 but is simple in the Pompilidae, Sphegidae, Vespidae, 

 Formicidae, and Apidae. Most of these Hymenoptera with 

 undivided trochanters (Fig. 77, c) have the ovipositor of the 

 female modified into a sting, and are therefore collectively 

 known as Aculeata. The true Wasps (Vespidae) are known 

 from the Pompilidae by the longitudinal folding of the 

 forewings when at rest. Ants are readily recognised by 

 the second (apparently first) abdominal segment, the 

 ** waist," forming a prominent *' node," and in many 

 groups of ants the third abdominal segment is similarly 

 modified. The Apidae or bees are distinguished by their 

 broad basal foot-segments and their plumose hairs ; 

 characters which are of great importance in connection 

 with their habit of gathering the pollen of flowers for food. 

 From these examples it can be seen that a family of insects 



