Fig. 28.
Nest (natural size) of Andrena vicina, showing the main burrow, and the cells leading from it; the oldest cell containing the pupa (a) is situated nearest the surface, while those containing the larva (b) lie between the pupa and the cell (e) containing the pollen mass and egg resting upon it. The most recent cell (f) is the deepest down, and contains a freshly deposited pollen mass. At c is the beginning of a cell; g, level of the ground.
A glance at the drawing (Fig. 28), of a burrow, with its side galleries, of the Andrena vicina, reveals the economy of one of our most common forms. Quite early in spring, when the sun and vernal breezes have dried up the soil, and the fields exchange their rusty hues for the rich green verdure of May, our Andrena, tired of its idle life among the blossoms of the willow, the wild cherry, and garden flowers, suddenly becomes remarkably industrious, and wields its spade-like jaws and busy feet with a strange and unwonted energy. Choosing some sunny, warm, grassy bank (these nests were observed in the "great pasture" of Salem), not always with a southern exposure however, the female sinks her deep well through the sod from six inches to a foot into the sandy soil beneath. She goes to work literally tooth and nail. Reasoning from observations made on several species of wasps, and also from studying the structure of her jaws and legs, it is evident that she digs in and loosens the soil with her powerful jaws, and then throws out the dirt with her legs. She uses her fore legs like hands, to pass the load of dirt to her hind legs, and then runs backward out of her hole to dump it down behind her. Mr. Emerton tells me that he never saw a bee in the act of digging but once, and then she left off after a few strokes. He also says, "they are harmless and inoffensive. On several occasions I have lain on the grass near their holes for hours, but not one attempted to sting me; and when taken between the fingers, they make but feeble resistance."
To enter somewhat into detail, we gather from the observations of Mr. Emerton (who has carefully watched the habits of these bees through several seasons) the following account of the economy of this bee: On the 4th of May the bees were seen digging their holes, most of which were already two inches deep, and one, six inches. The mounds of earth were so small as to be hardly noticed. At this time an Oil beetle was seen prowling about the holes. The presence of this dire foe of Andrena at this time, it will be seen in a succeeding chapter on the enemies of the bees, is quite significant. By the 15th of May, hundreds of Andrena holes were found in various parts of the pasture, and at one place, in a previous season, there were about two hundred found placed within a small area. One cell was dug up, but it contained no pollen. Four days later, several Andrenas were noticed resting from their toil at the opening of their burrows. On the 28th of May, in unearthing six holes, eight cells were found to contain pollen, and in two of them a small larva. The pellets of pollen are about the size of a small pea. They are hard and round at first, before the young has hatched, but as the larva grows, the mass becomes softer and more pasty, so that the larva buries its head in the mass, and greedily sucks it in. When is the pollen gathered by the bee and kneaded into the pellet-like mass? On July 4th, a cell was opened in which was a bee busily engaged preparing the pollen, which was loosely and irregularly piled up, while there was a larva in an adjoining cell nearly half an inch long. It would seem, then, that the bee comes in from the fields laden with her stores of pollen, which she elaborates into bee bread within her cell.
When the bee returns to her cell she does not directly fly towards the entrance, since, as was noticed in a particular instance, she flew about for a long time in all directions without any apparent aim, until she finally settled near the hole, and walked into her subterranean retreat. On a rainy day, May 24th, our friend visited the colony, but found no bees flying about the holes. The little hillocks had been beaten down by the pitiless raindrops, and all traces of their industry effaced. On digging down, several bees were found, indicating that on rainy days they seek the shelter of their holes, and do not take refuge under leaves of the plants they frequent.
On the 29th of June, six full-grown larvæ were exhumed, and one, about half grown. On the 20th of July, the colony seemed well organized, as, on laying open a burrow at the depth of six inches, he began to find cells. The upper ones, to the number of a dozen, were deserted and filled with earth and grass roots, and had evidently been built and used during the previous year. Below these were eight cells placed around the main vertical gallery, reaching down to the depth of thirteen inches, and all containing nearly full-grown larvæ of the bees, or else those of some parasitic bee (Nomada) which had devoured the food prepared for the young Andrena.
About the first of August the larva transforms to a pupa or chrysalis, as at this time two pupæ were found in cells a foot beneath the surface. As shown in the cut, those cells situated lowest down seem to be the last to have been made, while the eggs laid in the highest are the first to hatch, and the larvæ disclosed from them, the first to change to pupæ. Four days later the pupæ of Cuckoo bees (Nomada) were found in the cells. No Andrenas were seen flying about at this time.
On the 24th of August, to be still very circumstantial in our narrative though at the risk of being tedious, three burrows were unearthed, and in them three fully formed bees were found nearly ready to leave their cells, and in addition several pupæ. In some other cells there were three of the parasitic Nomada also nearly ready to come out, which seemed to be identical with some bees noticed playing very innocently about the holes early in the summer.
On the last day of August, very few of the holes were open. A number of Oil beetles were strolling suspiciously about in the neighborhood, and some little black Ichneumon flies were seen running about among the holes.
During mid-summer the holes were found closed night and day by clods of earth.
The burrow is sunken perpendicularly, with short passages leading to the cells, which are slightly inclined downwards and outwards from the main gallery. The walls of the gallery are rough, but the cells are lined with a mucous-like secretion, which, on hardening, looks like the glazing of earthenware. This glazing is quite hard, and breaks up into angular pieces. It is evidently the work of the bee herself, and is not secreted and laid on by the larva. The diameter of the interior of the cell is about one-quarter of an inch, contracting a little at the mouth. When the cell is taken out, the dirt adheres for a line in thickness, so that it is of the size and form of an acorn.