504 THE INVERTEBRATA 



Andrena constructs burrows in the ground and, though solitary, is 

 usually found in groups of individuals occupying a common terrain 

 which may include a "village " of several hundred nests. Nomada has 

 adopted the ** cuckoo" habit. 



Bombus enjoys a social existence similar to that of Vespa in that 

 only impregnated females survive the winter. 



The colony of the Honey bee Apis mellifica has more permanence, 

 only the males dying off in the autumn to leave the rest of the colony 

 to hibernate. The nest is of wax, an exudation from abdominal glands 

 of the worker (sterile female), and a material known as propolis of 

 vegetable origin serves to fasten parts of the nest together and to 

 render the whole weatherproof. 



The workers are graded according to age into nurses^ who see to the 

 welfare of the larvae by incorporating salivary juices with their food, 

 ventilators who by wing-fanning set up currents in the nest or hive 

 to reduce the temperature and to evaporate the honey, scavengers or 

 cleaners^ 2ind foragers who collect pollen and nectar. The changes from 

 nursery work to house work and to field work are necessitated by 

 changes in glandular capacity as age increases. Though the density 

 of the population of the colony determines to some extent when a 

 queen with a number of workers will depart from the hive as a 

 swarm, it appears that this event is also dependent on the relative 

 proportions of the above age-groups among the working caste .^ 



Order DIPTERA (Flies) 



Insects with a single pair of functional wings, the hind pair repre- 

 sented by stumps (halteres) (Fig. 347); mouth parts suctorial and 

 sometimes piercing or biting, usually elongated to form a proboscis ; 

 prothorax and metathorax small and fused with the large mesothorax ; 

 metamorphosis complete, larvae cruciform and always apodous, the 

 head frequently being reduced and retracted; pupa either free or 

 enclosed in the hardened larval skin (puparium). 



This is a very large and highly specialized order of insects. The 

 imagines are mostly diurnal species, feeding on the nectar of flowers, 

 but a number are predaceous, living on other insects (e.g. the robber 

 flies), while some, e.g.Tachinids, are parasites. A further development 

 which takes place in several families is the acquisition of blood- 



^ In Bees, Wasps and Ants, haploid parthenogenesis results in the pro- 

 duction of males. A fertilized (diploid) female has control over the fertiliza- 

 tion of eggs which she lays. If an egg is fertilized by sperm from the 

 spermatheca a female (diploid) offspring develops. If not, a male offspring 

 (haploid) develops. Whether the young female produced in the former case 

 becomes a worker (sterile) or a queen (capable of fertilization) depends on 

 nutrition. Contrast this with diploid parthenogenesis in Aphids (p. 480). 



