498 THE INVERTEBRATA 



lary glossae undoubtedly playing an important part in maintaining 

 a complete tube. The mandibles are now no longer biting organs but 

 tools used for manipulating material such as pollen and wax. Such 

 a feeding mechanism is the climax in an evolutionary process which 

 has involved in succession the fusion of the glossa lobes, as in the 

 sawtlies, the lengthening of the basal joints of the labium and maxilla 

 as in ColleteSy and finally the elongation of the glossa, e.g. Apis and 

 Bombus. 



The highly complex social life found in the bees, ants and wasps, 

 in which caste development is a feature of prime importance, is fore- 

 shadowed in the interesting behaviour of solitary wasps and bees. 

 The supply of food to the larva by progressive feeding , instead of mass 

 provisioning, appears to enable the parent to become acquainted with 

 its offspring, and this establishment of family life may be regarded 

 as the forerunner of the complex social state of higher forms .^ A 

 second important feature in the development of social life has been 

 the phenomenon of trophallaxis . Among wasps, for instance, the 

 worker taking food to a grub receives in turn a drop of saliva from 

 the grub. This is eagerly looked for by the workers, and it is suggested 

 that it is the mutual exchange of food between young and adult which 

 engenders in the adult an interest in the welfare of the colony. A 

 third important feature in social development has been the exploita- 

 tion of a particular form of food material which can be obtained in 

 large quantities, e.g. pollen and honey. 



The phenomenon of parasitism (Fig. 345) is highly developed in 

 the Hymenoptera; Ichneumons, Chalcids and Proctotrypids being 

 almost entirely parasitic. Almost all orders of insects are affected by 

 the activities of these very important insects, Qgg, larval, pupa, and 

 adult stages being parasitized. 



From the foregoing it will be seen that some of the most important 

 insects are included in this order. The sawflies are important as 

 agricultural pests. Flower- visiting bees are of great value in the 

 pollination of flowers. Carnivorous wasps do good by devouring 

 other insect pests such as aphides, while to a large extent the parasitic 

 Hymenoptera are useful in checking the depredations of phyto- 

 phagous insects. 



Two main types of larvae are found in this order, the legged larva 

 of the sawflies (Fig. 344 D) and the legless form of bees, wasps and 

 ants (Fig. 341 A). The sawfly larva has a superficial resemblance to 

 the lepidopterous caterpillar, but is distinguished by its single pair of 



^ In English species of the wasp Odynerus the egg is laid in a cell and 

 sufficient caterpillars stored to serve as food for the whole of the larval life 

 (mass provisioning). Certain African species of this genus supply their growing 

 larvae from day to day with fresh caterpillars (progressive feeding). 



