The species of the genus Psithyrus (Apathus of many authors) inhabit the nests of Bombus; although less numerous than the species of the latter genus, they also are widely distributed. They are so like Bombus in appearance that they were not distinguished from them by the earlier entomologists; and what is still more remarkable, each species of Psithyrus resembles the Bombus with which it usually lives. There appear, however, to be occasional exceptions to this rule, Smith having seen one of the yellow-banded Psithyrus in the nest of a red-tailed Bombus. Psithyrus is chiefly distinguished from Bombus by the absence of certain characters that fit the latter Insects for their industrial life; the hind tibiae have no smooth space for the conveyance of pollen, and, so far as is known, there are only two sexes, males and perfect females.
Fig. 23—Psithyrus vestalis, Britain. A, Female, x 3⁄2; B, outer side of hind leg.
The Bombus and Psithyrus live together on the best terms, and it appears probable that the latter do the former no harm beyond appropriating a portion of their food supplies. Schmiedeknecht says they are commensals, not parasites; but it must be admitted that singularly few descriptions of the habits and life-histories of these interesting Insects have been recorded. Hoffer has, however, made a few direct observations which confirm, and at the same time make more definite, the vague ideas that have been generally prevalent among entomologists. He found and took home a nest of Bombus variabilis, which contained also a female of Psithyrus campestris, so that he was able to make observations on the two. The Psithyrus was much less industrious than the Bombus, and only left the nest somewhat before noon, returning home again towards evening; after about a month this specimen became still more inactive, and passed entire days in the nest, occupying itself in consuming the stores of honey of its hosts, of which very large quantities were absorbed, the Psithyrus being much larger than the host-bee. The cells in which the young of the Psithyrus are hatched are very much larger than those of the Bombus, and, it may therefore be presumed, are formed by the Psithyrus itself, for it can scarcely be supposed that the Bombus carries its complaisance so far as to construct a cell specially adapted to the superior stature of its uninvited boarder. When a Psithyrus has been for some time a regular inhabitant of a nest, the Bombus take its return home from time to time as a matter of course, displaying no emotion whatever at its entry. Occasionally Hoffer tried the introduction of a Psithyrus to a nest that had not previously had one as an inmate. The new arrival caused a great hubbub among the Bombus, which rushed to it as if to attack it, but did not do so, and the alarm soon subsided, the Psithyrus taking up the position in the nest usually affected by the individuals of the species. On introducing a female Psithyrus to a nest of Bombus in which a Psithyrus was already present as an established guest, the latter asserted its rights and drove away the new comer. Hoffer also tried the experiment of placing a Psithyrus campestris in the nest of Bombus lapidarius—a species to which it was a stranger; notwithstanding its haste to fly away, it was at once attacked by the Bombus, who pulled it about but did not attempt to sting it.
When Psithyrus is present in a nest of Bombus it apparently affects the inhabitants only by diminishing their stores of food to so great an extent that the colony remains small instead of largely increasing in numbers. Although Bombus variabilis, when left to itself, increases the number of individuals in a colony to 200 or more, Hoffer found in a nest in which Psithyrus was present, that on the 1st of September the assemblage consisted only of a queen Bombus and fifteen workers, together with eighteen specimens of the Psithyrus, eight of these being females.
The nests of Bombus are destroyed by several animals, probably for the sake of the honey contained in the pots; various kinds of small mammals, such as mice, the weasel, and even the fox, are known to destroy them; and quite a fauna of Insects may be found in them; the relations of these to their hosts are very little known, but some undoubtedly destroy the bees' larvae, as in the case of Meloe, Mutilla and Conops. Birds do not as a rule attack these bees, though the bee-eater, Merops apiaster, has been known to feed on them very heavily.
The genera of social bees known as Melipona, Trigona or Tetragona, may, according to recent authorities, be all included in one genus, Melipona. Some of these Insects are amongst the smallest of bees, so that one, or more, species go by the name of "Mosquito-bees." The species appear to be numerous, and occur in most of the tropical parts of the continents of the world, but unfortunately very little is known as to their life-histories or economics; they are said to form communities consisting at times of a countless number of individuals; but it has not been thoroughly ascertained whether these are the produce of a single queen, as in the case of the hive-bee, or whether there may be more than one egg-producer in each community. The late F. Smith thought the former of these alternatives would prove to be correct. These mosquito-bees are frequently spoken of as stingless bees, but this is not quite correct, for although they do not sting, von Ihering[[34]] says that all the essential elements of the sting are present, the pointed or penetrating part of the apparatus being stunted.
It would serve no useful purpose to attempt to construct the social history of these stingless bees from the numerous brief scattered accounts in entomological literature, for they refer to different species; it is, however, positively stated by Smith on the authority of Peckolt[[35]] that Trigona mosquito sends off swarms after the manner of the hive-bee in this country, and that after searching six hives only one royal female could be found in each.
Fig. 24.—Melipona sp. ♀. Amazons.