The nests of Bombus are often shared by inquiline bumble bees of the genus Psithyrus, each species of the latter adopting a par ticular species of Bombus as its host. Furthermore, the colour and size of a particular inquiline bears a strikingly close resemblance to its host species of Bombus. The inquiline species have no worker caste and their queens are devoid of pollen-gathering structures. According to Sladen the Psithyrus stings the Bombus queen to death and usurps her place in the nest, the Bombus workers rearing the inquiline offspring along with the reduced numbers of their brood.
The stingless bees of the genera Melipona and Trigona number nearly 25o species, which are chiefly found in South America, rela tively few occurring in the tropics of the old world. The name stingless bees is a misnomer since a vestigial sting is present while, on account of their small size, they are sometimes called mos quito bees: when disturbed some kinds literally swarm over the face and in the hair of a human intruder to his great annoyance. The castes are well marked in these insects : the workers struc turally represent the typical females except that they are sterile, while the queen exhibits degeneration of the typical secondary characters of her sex. The colonies are monogynous but a num ber of daughter queens are tolerated and these, accompanied by detachments of workers, swarm from time to time and found new nests. The latter are found in tree hollows or on branches; more rarely they occur in the ground or in termites' nests. Most species mix their wax with earth or other material, thus producing a brown or black substance termed cerumen for comb-building.
Social life among bees reaches its highest development in the hive-bees or honey-bees of the genus Apis. Only four species are known, viz., A. mellifica the common hive-bee, together with the oriental species A. dorsata, indica and florea. The common hive bee (Apis mellifica) has been introduced into almost all countries of the world and is rarely, if ever, found wild in Britain. The three castes are very easily identified as the drone (male) is consider ably larger and stouter than the worker: it is readily distinguished by its blunter abdomen and the large eyes which occupy the greater part of the top of the head. The queen can be recog nized by her much longer abdomen, which extends some dis tance behind the closed wings. Her reproductive organs are completely developed and attain a large size, whereas in the workers these parts remain rudimentary or occasionally become partially developed in the case of fertile members of their own caste. The colonies are perennial and a flourishing hive will number 5o,000 to 8o,000 bees, the vast majority of them being workers. The combs are made of wax which is secreted between the ventral abdominal plates of the workers. Each of the combs is composed of a large number of typically hexagonal cells arranged in two series placed back to back: the separate combs hang vertically downwards with just enough space between them to admit of the free movements of the bees. Cells in
which workers develop are smaller than those destined to produce drones, while the largest are the royal cells, which are the cradles of the future queens. The royal cells are always very few in number, ovoid and irregular in form and are constructed by the workers as required. Other of the cells are used for storing honey and pollen which are drawn upon during the dormant months of the year. In addition to wax the workers utilize a resinous sub stance which they collect from the buds of poplar and other trees : this material is termed propollis and is used as a glue to fasten loose portions of the combs or for filling up crevices, etc. The queen lays an egg in each brood cell and the incubation period is about three days : the complete development of a queen occupies about 16 days, a worker three weeks and a drone 24 days. The fertilized eggs develop into workers or queens according to the type of feeding the larvae receive, and the unfertilized eggs pro duce drones. The young larvae, irrespective of caste, are nourished at first upon a food rich in protein, produced, it is believed, as a secretion of the workers' salivary glands. Queen larvae are fed upon this diet, which is commonly termed "royal jelly," until they pupate, while drone and worker larvae only receive this food up to the fourth day (or third day according to recent American observers) , after which they are nourished upon honey and predigested pollen. The queen is able to survive for several seasons, but performs no functions other than egg-laying and is capable of producing up to 1,500,000 eggs during her life : her legs are destitute of the pollen-collecting apparatus so well de veloped in the workers. New colonies are established by swarm ing and this event follows the emergence of a daughter queen. The latter remains in the hive and the swarm consists of the old queen accompanied by a host of workers. The new queen takes her marriage flight at the first opportunity and is followed by a number of drones : mating takes place in mid-air and the ferti lized queen then returns to the nest. If a second swarm be emitted the same season, it is accompanied not by the reigning queen, but by a new virgin queen. The worker bees when newly emerged are mainly active within the hive attending the brood, nest-build ing and performing other duties. As they become older they go out into the field in order to collect nectar and pollen: the latter duties are very exacting and summer-hatched workers seldom live longer than about six or seven weeks, while those hatched in autumn live to perform the labours of early spring.