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True Bees — Part 14

When Mr. Busck was making some studies for me in the summer of 1897, on the life history of the tussock moth, he made the interesting discovery that the egg masses of this famous shadetree defoliator were sometimes eaten by the larvae of certain dermestid beetles which are ordinarily known as museum pests, feeding upon skins and furs, stuffed birds and pinned insects, and which, although known to feed upon dead and dry animal matter, were hardly to be suspected of eating living animal matter. But we found (and this is by the way) that these museum pests were really destroying the sound eggs of the tussock moth. This interested us so much that egg masses with dermestid larvae were brought into the insectary for most careful observation. Then it was found that with the dermestids had also been brought in a most interesting parasite which proved to be Laelius trogodermatis--a Bethylid. The Laelius is a little, black, slender, active, four-winged fly; and the female, when it finds one of these dermestid larvae, at once jumps upon its back and clings firmly, in spite of the struggles of the victim. As soon as the poor beetle larva quiets down a bit, Laelius places herself crosswise over the thorax and, curling her abdomen around under the side, inserts her sting just behind the second or third pair of legs, paralyzing the dermestid instantly, the sting apparently having entered one of the large thoracic nerve ganglia. Then the parasite relaxes its hold and begins pulling the legs and hairs of its victim with its mandibles, its antennae vibrating in a contented manner. The pulling of the legs is evidently an attempt on the part of the parasite to see if the stinging has done its work with perfect effect. Having satisfied herself by all sorts of tests that the paralysis is complete, she proceeds to lay an egg, attaching it to the skin of the dermestid on the under side of the body, first pulling out the hairs carefully so that the egg can be firmly attached to the skin. If in the course of this operation, or even before the egg is laid, another dermestid larva comes within her range of perception, she leaves the first victim, mounts and stings the second, or even a third or a fourth, each time testing the completeness of the paralysis with the utmost care. Before attaching the egg she thrusts her sting into the spot several times, apparently making an orifice through which the larva, after hatching, can thrust its head, or which it can at least enlarge easily so as to insert its head. The egg is oval, soft, translucent, about a third of a millimeter long, apparently has no peduncle, and is not very firmly attached to the skin of the dermestid. From one to six eggs are laid upon a single victim. In a few days the larvae hatch, yellow in color and very indistinctly jointed. Immediately on hatching, their mouths are closely applied to the skin of the paralyzed dermestid and they begin to grow, not so rapidly as the somewhat similar Euplectrus, which will be described in a succeedingchapter, but still rather rapidly, reaching full growth in from ten to fourteen days. When full grown, a group of these larvae with their heads inserted at a central point, look not unlike the petals of a curious flower growing out of the shriveled dermestid larva. When only one Laelius larva occurs upon a host it sometimes enters the sucked-dry skin and spins its cocoon within it, but generally the white, rather loose, silken cocoons are spun outside the skin of the dermestid, which shows large holes where the parasitic larvae have been at work. After the cocoon is formed the larva remains within it, motionless, for ten days or more, finally transforming to a white pupa with red eyes. This white color changes gradually to black and in eight to ten days further the adult fly issues from the end of the cocoon through a ragged hole. The entire life duration of a generation, in the summer-time, is from thirty to thirty-five days and the insect passes the winter as a larva in its cocoon, the cocoons of the winter generation being firmer than those of the summer brood and darker in color. Mr. Busck observed that a female will paralyze all dermestid larvae with which it comes in contact, but it does not use them all for egg-laying. The sting does not kill the dermestid larva but simply paralyzes its motor nerves.