I have said nothing yet of the Chalicodoma of the Shrubs. My silence is not due to negligence, but to the circumstance that I am almost destitute of facts relating to her parasites. Of the many nests which I have opened in order to study their inhabitants, only one so far has been invaded by strangers. This nest, the size of a large walnut, was fixed on a pomegranate-branch. It comprised eight cells, of which seven were occupied by the Chalicodoma, and the eighth by a little Chalcis, the plague of a whole host of the Bee-tribe. Apart from this instance, which was not a very serious case, I have seen nothing. In those aerial nests, swinging at the end of a twig, not a Dioxys, a Stelis, an Anthrax, a Leucopsis, those dread ravagers of the other two Masons; never any Osmiae, Megachiles or Anthidia, those lodgers in the old buildings.
The absence of the latter is easily explained. The Chalicodoma's masonry does not last long on its frail support. The winter winds, when the shelter of the foliage has disappeared, must easily break the twig, which is little thicker than a straw and liable to give way by reason of its heavy burden. Threatened with an early fall, if it is not already on the ground, last year's dwelling is not restored to serve the needs of the present generation. The same nest does not serve twice; and this does away with the Osmiae and with their rivals in the art of utilizing old cells.
The elucidation of this point does not remove the obscurity of the next. I can see nothing to account for the absence or at least the extreme rareness of usurpers of provisions and consumers of grubs, both of whom are very indifferent to the new or old conditions of the nest, so long as the cells are well stocked. Can it be that the lofty position of the edifice and the shaky support of the twig arouse distrust in the Dioxys and other malefactors? For lack of a better explanation, I will leave it at that.
If my idea is not an empty fancy, we must admit that the Chalicodoma of the Shrubs was singularly well-inspired in building in mid-air. You have seen of what misfortunes the other two are victims. If I take a census of the population of a tile, many a time I find the Dioxys and the Mason-bee in almost equal proportions. The parasite has wiped out half the colony. To complete the disaster, it is not unusual for the grub-eaters, the Leucopsis and her rival, the pygmy Chalcis, to have decimated the other half. I say nothing of Anthrax sinuata, whom I sometimes see coming from the nests of the Chalicodoma of the Sheds; her larva preys on the Three-horned Osmia, the Mason-bee's visitor.
All solitary though she be on her boulder, which would seem the proper thing to keep away exploiters, the scourge of dense populations, the Chalicodoma of the Pebbles is no less sorely tried. My notes abound in cases such as the following: of the nine cells in one dome, three are occupied by the Anthrax, two by the Leucopsis, two by the Stelis, one by the Chalcis and the ninth by the Mason. It is as though the four miscreants had joined forces for the massacre: the whole of the Bee's family has disappeared, all but one young mother saved from the disaster by her position in the centre of the citadel. I have sometimes stuffed my pockets with nests removed from their pebbles without finding a single one that has not been violated by one or other of the malefactors and oftener still by several of them at a time. It is almost an event for me to find a nest intact. After these funereal records, I am haunted by a gloomy thought: the weal of one means the woe of another.
CHAPTER 11. THE LEUCOPSES.
(This chapter should be read in conjunction with the essays entitled "The Anthrax" and "Larval Dimorphism", forming chapters 2 and 4 of "The Life of the Fly."—Translator's Note.)
Let us visit the nests of Chalicodoma muraria in July, detaching them from their pebbles with a sideward blow, as I explained when telling the story of the Anthrax. The Mason-bee's cocoons with two inhabitants, one devouring, the other in process of being devoured, are numerous enough to allow me to gather some dozens in the course of a morning, before the sun becomes unbearably hot. We will give a smart tap to the flints so as to loosen the clay domes, wrap these up in newspapers, fill our box and go home as fast as we can, for the air will soon be as fiery as the devil's kitchen.
Inspection, which is easier in the shade indoors, soon tells us that, though the devoured is always the wretched Mason-bee, the devourer belongs to two different species. In the one case, the cylindrical form, the creamy-white colouring and the little nipple constituting the head reveal to us the larva of the Anthrax, which does not concern us at present; in the other, the general structure and appearance betray the grub of some Hymenopteron. The Mason's second exterminator is, in fact, a Leucopsis (Leucopsis gigas, FAB.), a magnificent insect, stripped black and yellow, with an abdomen rounded at the end and hollowed out, as is also the back, into a groove to contain a long rapier, as slender as a horsehair, which the creature unsheathes and drives through the mortar right into the cell where it proposes to establish its egg. Before occupying ourselves with its capacities as an inoculator, let us learn how its larva lives in the invaded cell.