Fabre states that M. albocincta, which commences the formation of its nest in a worm-burrow by means of a barricade, frequently makes the barricade, but no nest; sometimes it will indeed make the barricade more than twice the proper size, and thus completely fill up the worm burrow. Fabre considers that these eccentric proceedings are due to individuals that have already formed proper nests elsewhere, and that after completing these have still some strength remaining, which they use up in this fruitless manner.
The Social bees (Sociales) include, so far as is yet known, only a very small number of genera, and are so diverse, both in habits and structure, that the propriety of associating them in one group is more than doubtful; the genera are Bombus (Fig. 331, vol. v.), with its commensal genus or section, Psithyrus (Fig. 23); Melipona (Fig. 24), in which Trigona and Tetragona may at present be included, and Apis (Fig. 6); this latter genus comprising the various honey-bees that are more or less completely domesticated in different parts of the world.
In the genus Bombus the phenomena connected with the social life are more similar to what we find among wasps than to what they are in the genus Apis. The societies come to an end at the close of the season, a few females live through the winter, and each of these starts a new colony in the following spring. Males, females and workers exist, but the latter are not distinguished by any good characters from the females, and are, in fact, nothing but more or less imperfect forms thereof; whereas in Apis the workers are distinguished by structural characters not found in either of the true sexes.
Hoffer has given a description of the commencement of a society of Bombus lapidarius.[[31]] A large female, at the end of May, collected together a small mass of moss, then made an expedition and returned laden with pollen; under cover of the moss a cell was formed of wax taken from the hind-body and mixed with the pollen the bee had brought in; this cell was fastened to a piece of wood; when completed it formed a subspherical receptacle, the outer wall of which consisted of wax, and whose interior was lined with honey-saturated pollen; then several eggs were laid in this receptacle, and it was entirely closed. Hoffer took the completed cell away to use it for museum purposes, and the following day the poor bee that had formed it died. From observations made on Bombus agrorum he was able to describe the subsequent operations; these are somewhat as follows:—The first cell being constructed, stored, and closed, the industrious architect, clinging to the cell, takes a few days' rest, and after this interval commences the formation of a second cell; this is placed by the side of the first, to which it is connected by a mixture of wax and pollen; the second cell being completed a third may be formed; but the labours of the constructor about this time are augmented by the hatching of the eggs deposited a few days previously; for the young larvae, having soon disposed of the small quantity of food in the interior of the waxen cell, require feeding. This operation is carried on by forming a small opening in the upper part of the cell, through which the bee conveys food to the interior by ejecting it from her mouth through the hole; whether the food is conveyed directly to the mouths of the larvae or not, Hoffer was unable to observe; it being much more difficult to approach this royal founder without disturbing her than it is the worker-bees that carry on similar occupations at a subsequent period in the history of the society. The larvae in the first cell, as they increase in size, apparently distend the cell in an irregular manner, so that it becomes a knobbed and rugged, truffle-like mass. The same thing happens with the other cells formed by the queen. Each of these larval masses contains, it should be noticed, sister-larvae all of one age; when full grown they pupate in the mass, and it is worthy of remark that although all the eggs in one larval mass were laid at the same time, yet the larvae do not all pupate simultaneously, neither do all the perfect Insects appear at once, even if all are of one sex. The pupation takes place in a cocoon that each larva forms for itself of excessively fine silk. The first broods hatched are formed chiefly, if not entirely, of workers, but small females may be produced before the end of the season. Huber and Schmiedeknecht state that though the queen provides the worker-cells with food before the eggs are placed therein, yet no food is put in the cells in which males and females are produced. The queen, at the time of pupation of the larvae, scrapes away the wax by which the cocoons are covered, thus facilitating the escape of the perfect Insect, and, it may also be, aiding the access of air to the pupa. The colony at first grows very slowly, as the queen can, unaided, feed only a small number of larvae. But after she receives the assistance of the first batch of workers much more rapid progress is made, the queen greatly restricting her labours, and occupying herself with the laying of eggs; a process that now proceeds more and more rapidly, the queen in some cases scarcely ever leaving the nest, and in others even becoming incapable of flight. The females produced during the intermediate period of the colony are smaller than the mother, but supplement her in the process of egg-laying, as also do the workers to a greater or less extent. The conditions that determine the egg-laying powers of these small females and workers are apparently unknown, but it is ascertained that these powers vary greatly in different cases, so that if the true queen die the continuation of the colony is sometimes effectively carried on by these her former subordinates. In other cases, however, the reverse happens, and none of the inhabitants may be capable of producing eggs: in this event two conditions may be present; either larvae may exist in the nest, or they may be absent. In the former case the workers provide them with food, and the colony may thus still be continued; but in the latter case, there being no profitable occupation for the bees to follow, they spend the greater part of the time sitting at home in the nest.
Supposing all to go well with the colony it increases very greatly, but its prosperity is checked in the autumn; at this period large numbers of males are produced as well as new queens, and thereafter the colony comes to an end, only a few fertilised females surviving the winter, each one to commence for herself a new colony in the ensuing spring.
The interior of the nest of a bumble-bee (Bombus) frequently presents a very irregular appearance; this is largely owing to the fact that these bees do not use the cells as cradles twice, but form others as they may be required, on the old remains. The cells, moreover, are of different sizes, those that produce workers being the smallest, those that cradle females being the largest, while those in which males are reared are intermediate in size. Although the old cells are not used a second time for rearing brood they are nevertheless frequently adapted to the purposes of receptacles for pollen and for honey, and for these objects they may be increased in size and altered in form.
It may be gathered from various records that the period required to complete the development of the individual Bombus about midsummer is four weeks from the deposition of the egg to the emergence of the perfect Insect, but exact details and information as to whether this period varies with the sex of the Insect developed are not to be found. The records do not afford any reason for supposing that such distinction will be found to exist: the size of the cells appears the only correlation, suggested by the facts yet known, between the sex of the individual and the circumstances of development.
The colonies of Bombus vary greatly in prosperity, if we take as the test of this the number of individuals produced in a colony. They never, however, attain anything at all approaching to the vast number of individuals that compose a large colony of wasps, or that exist in the crowded societies of the more perfectly social bees. A populous colony of a subterranean Bombus may attain the number of 300 or 400 individuals. Those that dwell on the surface are as a rule much less populous, as they are less protected, so that changes of weather are more prejudicial to them. According to Smith, the average number of a colony of B. muscorum in the autumn in this country is about 120—viz. 25 females, 36 males, 59 workers. No mode of increasing the nests in a systematic manner exists in this genus; they do not place the cells in stories as the wasps do; and this is the case notwithstanding the fact that a cell is not twice used for the rearing of young. When the ground-space available for cell-building is filled the Bombus begins another series of cells on the ruins of the first one. From this reason old nests have a very irregular appearance, and this condition of seeming disorder is greatly increased by the very different sizes of the cells themselves. We have already alluded to some of these cells, more particularly to those of different capacities to suit the sexes of the individuals to be reared in them. In addition to these there are honey-tubs, pollen-tubs, and the cells of the Psithyrus (Fig. 23), the parasitic but friendly inmates of the Bombus-nests. A nest of Bombus, exhibiting the various pots projecting from the remains of empty and partially destroyed cells, presents, as may well be imagined, a very curious appearance. Some of the old cells apparently are partly destroyed for the sake of the material they are composed of. Others are formed into honey-tubs, of a make-shift nature. It must be recollected that, as a colony increases, stores of provisions become absolutely necessary, otherwise in bad weather the larvae could not be fed. In good weather, and when flowers abound, these bees collect and store honey in abundance; in addition to placing it in the empty pupa-cells, they also form for it special receptacles; these are delicate cells made entirely of wax filled with honey, and are always left open for the benefit of the community. The existence of these honey-tubs in bumble-bees' nests has become known to our country urchins, whose love for honey and for the sport of bee-baiting leads to wholesale destruction of the nests. According to Hoffer, special tubs for the storing of pollen are sometimes formed; these are much taller than the other cells. The Psithyrus that live in the nests with the Bombus are generally somewhat larger than the latter, and consequently their cells may be distinguished in the nests by their larger size. A bumble-bees' nest, composed of all these heterogenous chambers rising out of the ruins of former layers of cells, presents a scene of such apparent disorder that many have declared that the bumble-bees do not know how to build.
Although the species of Bombus are not comparable with the hive-bee in respect of the perfection and intelligent nature of their work, yet they are very industrious Insects, and the construction of the dwelling-places of the subterranean species is said to be carried out in some cases with considerable skill, a dome of wax being formed as a sort of roof over the brood cells. Some work even at night. Fea has recorded the capture of a species in Upper Burmah working by moonlight, and the same industry may be observed in this country if there be sufficient heat as well as light. Godart, about 200 years ago, stated that a trumpeter-bee is kept in some nests to rouse the denizens to work in the morning: this has been treated as a fable by subsequent writers, but is confirmed in a circumstantial manner by Hoffer, who observed the performance in a nest of B. ruderatus in his laboratory. On the trumpeter being taken away its office was the following morning filled by another individual The trumpeting was done as early as three or four o'clock in the morning, and it is by no means impossible that the earliness of the hour may have had something to do with the fact that for 200 years no one confirmed the old naturalist's observation.
One of the most curious facts in connection with Bombus is the excessive variation that many of the species display in the colour of the beautiful hair with which they are so abundantly provided. There is not only usually a difference between the sexes in this respect, but also extreme variation within the limits of the same sex, more especially in the case of the males and workers; there is also an astonishing difference in the size of individuals. These variations are carried to such an extent that it is almost impossible to discriminate all the varieties of a species by inspection of the superficial characters. The structures peculiar to the male, as well as the sting of the female, enable the species to be determined with tolerable certainty. Cholodkovsky,[[32]] on whose authority this statement as to the sting is made, has not examined it in the workers, so that we do not know whether it is as invariable in them as he states it to be in queens of the same species. According to Handlirsch,[[33]] each species of Bombus has the capacity of variation, and many of the varieties are found in one nest, that is, among the offspring of a single pair of the species, but many of the variations are restricted to certain localities. Some of the forms can be considered as actual ("fertige") species, intermediate forms not being found, and even the characters by which species are recognised being somewhat modified. As examples of this he mentions Bombus silvarum and B. arenicola, B. pratorum and B. scrimshiranus. In other cases, however, the varieties are not so discontinuous, intermediate forms being numerous; this condition is more common than the one we have previously described; B. terrestris, B. hortorum, B. lapidarius and B. pomorum are examples of these variable species. The variation runs to a considerable extent in parallel lines in the different species, there being a dark and a light form of each; also each species that has a white termination to the body appears in a form with a red termination, and vice versa. In the Caucasus many species that have everywhere else yellow bands possess them white; and in Corsica there are species that are entirely black, with a red termination to the body, though in continental Europe the same species exhibit yellow bands and a white termination to the body. With so much variation it will be readily believed that much remains to be done in the study of this fascinating genus. It is rich in species in the Northern hemisphere, but poor in the Southern one, and in both the Ethiopian and Australian regions it is thought to be entirely wanting.