It is hard to tell the Resin-bees’ nests, because the insect often makes its home at the very inside of the spiral, a long way from the mouth. I hold up a shell to the light. If it is quite transparent, I know that it is empty and I put it back to be used for future nests. If the second whorl is opaque, does not let the light through, the spiral contains something. What? Earth washed in by the rain? Remnants of the dead Snail? That remains to be seen. With a little pocket-trowel I make a wide window in the middle of the final whorl. If I see a gleaming resin floor, with incrustations of gravel, the thing is settled: I have a Resin-bee’s nest.
The Bee picks out the particular whorl of the shell which is the right size for her nest. In large shells, the nest is near the back; in smaller shells, at the very front, where the passage is widest. She always makes a partition of a mosaic formed of bits of gravel set in gum. I did not know at first what this gum was. It is amber-colored, semi-transparent, brittle, soluble in spirits of wine, and burns with a sooty flame and a strong smell of resin. These characteristics told me that the Bee uses the resinous drops that ooze from the trunks of various cone-bearing trees. There are plenty of junipers in the neighborhood, and I think that these form the main part of this Bee’s materials. If there were pines, cypresses, and other cone-bearing trees near, she would probably use those.
After the lid of resin and gravel, the Bee stops up the shell still further with bits of gravel, catkins and needles of the juniper, and other odds and ends, including a few rare little land-shells. This is the secondary barrier, to make the shell still safer for her nest. The Cotton-bee uses the same sort of barrier in the bramble. The Resin-bee uses it only in the larger shells, where there is much vacant space; in the smaller ones, where her nest reaches nearly to the entrance, she does without it.
The cells come next, farther back in the spiral. There are usually only two. The front room, which is the larger, contains a male, which in this kind of Bee is larger than the female; the smaller back room houses a female. It is extraordinary how the mother Bee knows the sex of the egg she is laying. This matter has never been explained to the satisfaction of scientists.
The Resin-bee makes a mistake in choosing large shells and not filling them up to the very entrance. The Osmia-bee also makes her nest in snail-shells; she often seizes upon the empty rooms in the Resin-bee’s house and fills them with her mass of cells. She then stops up the entrance with a thick clay stopper. When July comes, this house with the two families of tenants becomes the scene of a tragic conflict. The Resin-bees, in the back rooms, on attaining the adult state, burst their swaddling bands, bore their way through the resin partitions, pass through the gravel barricade and try to release themselves. Alas, the strange family ahead blocks the way! The Osmia inmates are still in the grub stage; they mean to stay in their cells till the next spring. The Resin-bees cannot get out through this second row of clay-stoppered cells; they give up all hope and perish behind the wall of earth. If their mother had only foreseen this danger, the disaster would never have happened; but instinct has failed her for once. Misfortune has not taught the Resin-bees anything through all the generations; and this contradicts the theory of those scientists who say that animals learn through experience.