Fig. 11.—Sphecodes gibbus ♀. Britain.
The great majority of the species of short-tongued bees found in Britain belong to the genera Andrena and Halictus, and with some others constitute the Andrenides of many writers. Halictus includes our smallest British bees. Their economy escaped the earlier observers, but has recently been to some extent unravelled by Smith, Fabre, Nicolas, Verhoeff, and others, and proves to be of great interest and variety. Fabre observed H. lineolatus and H. sexcinctus[[23]] under circumstances that enabled him to give them continuous attention, whenever requisite, throughout a whole year. These bees are to a certain extent social; they are gregarious; each bee works for its own progeny, but there is collaboration between members of a colony, inasmuch as a piece of general work is undertaken from which more families than one derive benefit. This common work is a gallery, that, ramifying in the earth, gives access to various groups of cells, each group the production of a single Halictus; in this way one entrance and one corridor serve for several distinct dwellings. The work of excavation is carried on at night. The cells are oval, and are covered on the interior with a delicate waterproof varnish; Fabre considers this to be a product of the salivary glands, like the membrane we noticed when speaking of Colletes. In the south of France both sexes of these species are produced from the nests in September, and then the males are much more numerous than the females; when the cold weather sets in the males die, but the females continue to live on in the cells underground. In the following spring the females come out and recommence working at the burrows, and also provision the cells for the young; the new generation, consisting entirely of females, appears in July, and from these there proceeds a parthenogenetic generation, which assumes the perfect form in September, and consists, as we have above remarked, in greater part of males. Pérez,[[24]] however, considers that Fabre's observations as to the parthenogenetic generation were incomplete, and that males might have been found a little earlier, and he consequently rejects altogether the occurrence of parthenogenesis in Halictus. Nicolas confirms Fabre's observations, so far as the interesting point of the work done for common benefit is concerned; and adds that the common corridor being too narrow to permit of two bees passing, there is a dilatation or vestibule near the entrance that facilitates passage, and also that a sentinel is stationed at this point.
Smith's observations on Halictus morio in England lead one to infer that there is but one generation, the appearance of which extends over a very long period. He says, "Early in April the females appeared, and continued in numbers up to the end of June"; then there was an interval, and in the middle of August males began to appear, followed in ten or twelve days by females. Hence it is probable that in different countries the times of appearance and the number of generations of the same species may vary. Verhoeff has described the burrows of Halictus quadricinctus with some detail. The cells, instead of being distributed as usual throughout the length of the burrow one by one, are accumulated into a mass placed in a vault communicating with the shaft. This shaft is continued downwards to a depth of 10 cm., and forms a retreat for the bees when engaged in construction. Several advantages are secured by this method, especially better ventilation, and protection from any water that may enter the shaft. The larvae that are present in the brood-chambers at any one moment differ much in their ages, a fact that throws some doubt on the supposed parthenogenetic generation. No cocoons are formed by these Halictus, the polished interior of the cell being a sufficiently refined resting place for metamorphosis. Verhoeff states that many of the larvae are destroyed by mouldiness; this indeed, he considers to be the most deadly of the enemies of Aculeate Hymenoptera. The nest of Halictus maculatus has also been briefly described by Verhoeff, and is a very poor construction in comparison with that of H. quadricinctus.
Fig. 12—Nesting of Halictus quadricinctus. u, Original burrow, with entrance e thereto; n, retreat or continuation of the burrow; w, the vaults; s, the accumulation of cells. (After Verhoeff, Verh. Ver. Rheinl. xlviii. 1891; scale not mentioned.)
The genus Andrena includes a great number of species, Britain possessing about fifty. They may be described in a general manner as Insects much resembling the honey-bee—for which, indeed, they are frequently mistaken—but usually a little smaller in size. Many of the bees we see in spring, in March or April, are of this genus. They live in burrows in the ground, preferring sandy places, but frequently selecting a gravel path as the locality for their operations; they nearly always live in colonies. Great difficulties attend their study on account of several points in their economy, such as, that the sexes are different, and frequently not found together; also that there may be two generations of a species in one year, these being more or less different from one another. Another considerable difficulty arises from the fact that these bees are subject to the attacks of the parasite Stylops, by which their form is more or less altered. These Insects feed in the body of the bee in such a way as to affect its nutrition without destroying its life; hence they offer a means of making experiments that may throw valuable light on obscure physiological questions. Among the effects they produce in the condition of the imago bee we may mention the enfeeblement of the sexual distinction, so that a stylopised male bee becomes less different than it usually is from the female, and a stylopised female may be ill developed and less different than usual from the male. The colours and hair are sometimes altered, and distortion of portions of the abdominal region of the bee are very common. Further particulars as to these parasites will be found at the end of our account of Coleoptera (p. [298]). We may here remark that these Stylops are not the only parasitic Insects that live in the bodies of Andrenidae without killing their hosts, or even interrupting their metamorphoses. Mr. R. C. L. Perkins recently captured a specimen of Halictus rubicundus, from which he, judging from the appearance of the example, anticipated that a Stylops would emerge; but instead of this a Dipterous Insect of the family Chloropidae appeared. Dufour in 1837 called attention to a remarkable relation existing between Andrena aterrima and a parasitic Dipterous larva. The larva takes up a position in the interior of the bee's body so as to be partly included in one of the great tracheal vesicles at the base of the abdomen; and the bee then maintains the parasite in its position, and at the same time supplies it with air by causing two tracheae to grow into its body. Dufour states that he demonstrated the continuity of the tracheae of the two organisms, but it is by no means clear that the continuity was initially due to the bee's organisation.
Fig. 13—Parasitic Dipterous larva in connection with tracheal system of Andrena aterrima. (After Dufour.)
Fig. 14.—D. hirtipes ♀. Britain.