Dasypoda hirtipes appears to be the most highly endowed of the European Andrenides. The Insects of the genus Dasypoda are very like Andrena, but have only two in place of three submarginal cells (just beneath the stigma) on the front wing. The female of D. hirtipes has a very dense and elongate pubescence on the posterior legs, and carries loads of pollen, each about half its own weight, to its nest. The habits of this insect have been described by Hermann Müller.[[25]] It forms burrows in the ground after the fashion of Andrena; this task is accomplished by excavating with the mandibles; when it has detached a certain quantity of the earth it brings this to the surface by moving backwards, and then distributes the loose soil over a considerable area. It accomplishes this in a most beautiful manner by means of the combined action of all the legs, each pair of these limbs performing its share of the function in a different manner; the front legs acting with great rapidity—making four movements in a second—push the sand backwards under the body, the bee moving itself at the same time in this direction by means of the middle pair of legs; simultaneously, but with a much slower movement, the hind legs are stretched and moved outwards, in oar-like fashion, from the body, and thus sweep away the earth and distribute it towards each side. This being done the bee returns quickly into the hole, excavates some more earth, brings it up and distributes it. Each operation of excavation takes a minute or two, the distribution on the surface only about fifteen seconds. The burrow extends to the length of one or two feet, so that a considerable amount of earth has to be brought up; and when the Insect has covered one part of the circumference of the mouth of the hole with loose earth, it makes another patch, or walk, by the side of the first. The main burrow being completed, the Insect then commences the formation of brood-chambers in connection with it. Three to six such chambers are formed in connection with a burrow; the lower one is first made and is provisioned by the bee: for this purpose five or six loads of pollen are brought to the cell, each load being, as we have already remarked, about half the weight of the Insect. This material is then formed into a ball and made damp with honey; then another load of pollen is brought, is mixed with honey and added as an outer layer to the ball, which is now remodelled and provided on one side with three short feet, after which an egg is placed on the top of the mass; the bee then sets to work to make a second chamber, and uses the material resulting from the excavation of this to close completely the first chamber. The other chambers are subsequently formed in a similar manner, and then the burrow itself is filled up. While engaged in ascertaining these facts, Müller also made some observations on the way the bee acts when disturbed in its operations, and his observations on this point show a very similar instinct to that displayed by Chalicodoma, referred to on a subsequent page. If interrupted while storing a chamber the Insect will not attempt to make a fresh one, but will carry its stock of provisions to the nest of some other individual. The result of this proceeding is a struggle between the two bees, from which it is satisfactory to learn that the rightful proprietor always comes out victorious. The egg placed on the pollen-ball in the chamber hatches in a few days, giving birth to a delicate white larva of curved form. This creature embraces the pollen-ball so far as its small size will enable it to do so, and eats the food layer by layer so as to preserve its circular form. The larva when hatched has no anal orifice and voids no excrement, so that its food is not polluted; a proper moulting apparently does not take place, for though a new delicate skin may be found beneath the old one this latter is not definitely cast off. When the food, which was at first 100 to 140 times larger than the egg or young larva, is all consumed the creature then for the first time voids its refuse. During its growth the larva becomes red and increases in weight from .0025 grains to .26 or .35 grains, but during the subsequent period of excretion it diminishes to .09 or .15 grains, and in the course of doing so becomes a grub without power of movement, and of a white instead of a red colour. After this the larva reposes motionless for many months—in fact, until the next summer, when it throws off the larval skin and appears as a pupa. The larval skin thus cast off contrasts greatly with the previous delicate condition of the integument, for this last exuvium is thick and rigid. Although it voids no excrement till much later the union of the stomach and hind-intestine is accomplished when the larva is half-grown. A larva, from which Müller took away a portion of its unconsumed food-store, began directly afterwards to emit excrement. The pupa has greater power of movement than the resting larva; when it has completed its metamorphosis and become a perfect Insect, it, if it be a female, commences almost immediately after its emergence to form burrows by the complex and perfect series of actions we have described.
Parasitic Bees (Denudatae).—This group of parasitic bees includes fourteen European genera, of which six are British. They form a group taxonomically most unsatisfactory, the members having little in common except the negative characters of the absence of pollen-carrying apparatus. Although there is a great dearth of information as to the life-histories of parasitic bees, yet some highly interesting facts and generalisations about their relations with their hosts have already been obtained. Verhoeff has recently given the following account of the relations between the parasitic bee Stelis minuta and its host Osmia leucomelana:—The Osmia forms cells in blackberry stems, provisions them in the usual manner, and deposits an egg in each. But the Stelis lays an egg in the store of provisions before the Osmia does, and thus its egg is placed lower down in the mass of food than that of the legitimate owner, which is in fact at the top. The Stelis larva emerges from the egg somewhat earlier than the Osmia larva does. For a considerable time the two larvae so disclosed consume together the stock of provisions, the Osmia at the upper, the Stelis at the lower, end thereof. By the consumption of the provisions the two larvae are brought into proximity, and by this time the Stelis larva, being about twice the size of the Osmia larva, kills and eats it. Verhoeff witnessed the struggle between the two larvae, and states further that the operation of eating the Osmia larva after it has been killed lasts one or two days. He adds that parasitic larvae are less numerous than the host larvae, it being well known that parasitic bees produce fewer offspring than host bees. Verhoeff further states that he has observed similar relations to obtain between the larvae of other parasitic bees and their hosts, but warns us against concluding that the facts are analogous in all cases.
Fig. 15.—Nomada sex-fasciata ♀. Britain.
Fabre has made us acquainted with some points in the history of another species of the same genus, viz. Stelis nasuta, that show a decided departure from the habits of S. minuta. The first-named Insect accomplishes the very difficult task of breaking open the cells of the mason-bee, Chalicodoma muraria, after they have been sealed up, and then, being an Insect of much smaller size than the Chalicodoma, places several eggs in one cell of that bee. Friese informs us that parasitic bees and their hosts, in a great number of cases, not only have in the perfect state the tongue similarly formed, but also frequent the same species of flower; thus Colletes daviesanus and its parasite Epeolus variegatus both specially affect the flowers of Tanacetum vulgare. Some of the parasitic bees have a great resemblance to their hosts; Stelis signata, for instance, is said to be so like Anthidium strigatum that for many years it was considered to be a species of the genus Anthidium. In other cases not the least resemblance exists between the parasites and hosts. Thus the species of Nomada that live at the expense of species of the genus Andrena have no resemblance thereto. Friese further tells us that the Andrena and Nomada are on the most friendly terms. Andrena, as is well known, forms populous colonies in banks, paths, etc., and in these colonies the destroying Nomada flies about unmolested; indeed, according to Friese, it is treated as a welcome guest. He says he has often seen, and in several localities, Nomada lathburiana and Andrena ovina flying peacefully together. The Nomada would enter a burrow, and if it found the Andrena therein, would come out and try another burrow; if when a marauding Nomada was in a burrow, and the rightful owner, returning laden with pollen, found on entering its home that an uninvited guest was therein, the Andrena would go out in order to permit the exit of the Nomada, and then would again enter and add the pollen to the store. Strange as this may seem at first sight, it is really not so, for, as we have before had occasion to observe, there is not the slightest reason for believing that host Insects have any idea whatever that the parasites or inquilines are injurious to their race. Why then should they attack the creatures? Provided the parasites do not interfere in any unmannerly way with the hosts and their work, there is no reason why the latter should resent their presence. The wild bee that seals up its cell when it has laid an egg therein, and then leaves it for ever, has no conception of the form of its progeny; never in the history of the race of the Andrena has a larva seen a perfect insect and survived thereafter, never has a perfect Insect seen a larva. There is no reason whatever for believing that these Insects have the least conception of their own metamorphosis, and how then should they have any idea of the metamorphosis of the parasite? If the Andrena found in the pollen the egg of a parasitic Nomada, it could of course easily remove the egg; but the Andrena has no conception that the presence of the egg ensures the death of its own offspring and though the egg be that of an enemy to its race, why should it resent the fact? Is it not clear that the race has always maintained itself notwithstanding the enemy? Nature has brought about that both host and parasite should successfully co-exist; and each individual of each species lives, not for itself, but for the continuance of the species; that continuance is provided for by the relative fecundities of host and guest. Why then should the Andrena feel alarm? If the species of Nomada attack the species of Andrena too much it brings about the destruction of its own species more certainly than that of the Andrena.
Fig. 16.—Melecta luctuosa ♀. Britain.
Such extremely friendly relations do not, however, exist between all the parasitic bees and their hosts. Friese says that, so far as he has been able to observe, the relations between the two are not in general friendly. He states that marauders of the genera Melecta and Coelioxys seek to get out of the way when they see the pollen-laden host coming home. But he does not appear to have noted any other evidence of mistrust between the two, and it is somewhat doubtful whether this act can properly be interpreted as indicating fear, for bees, as well as other animals, when engaged in work find it annoying to be interfered with; it is the interest of the parasite to avoid annoyance and to be well-mannered in its approaches. Shuckard, however, says that battles ensue between the parasite Melecta and its host Anthophora, when the two bees meet in the burrows of the Anthophora.[[26]]
We shall have occasion to remark on some of the habits of Dioxys cincta when considering the history of the mason-bee (Chalicodoma), but one very curious point in its economy must here be noticed. The Dioxys, which is a much smaller bee than the Chalicodoma, lays an egg in a cell of the latter, and the resulting larva frequently has more food in the cell than it can consume; there is, however, another bee, Osmia cyanoxantha, that frequently takes advantage of an unoccupied cell in the nest of the Chalicodoma, and establishes its own offspring therein. The Dioxys, it seems, cannot, or at any rate does not, distinguish whether a cell is occupied by Chalicodoma or by Osmia, and sometimes lays its egg in the nest of the Osmia, though this bee is small, and therefore provides very little food for its young. It might be supposed that under these conditions the Dioxys larva would be starved to death; but this is not so; it has the power of accommodating its appetite, or its capacity for metamorphosis, to the quantity of food it finds at its disposal, and the egg laid in the Osmia cell actually produces a tiny specimen of Dioxys, only about half the natural size. Both sexes of these dwarf Dioxys are produced, offering another example of the fact that the quantity of food ingested during the lifetime of the larva does not influence the sex of the resulting imago.
The highly endowed bees that remain to be considered are by some writers united in a group called Apidae, in distinction from Andrenidae. For the purposes of this work we shall adopt three divisions, Scopulipedes, Dasygastres, Sociales.