The particular method of attack and capture practiced by each species in securing its prey is instinctive. Ammophila pricks a number of ganglia along the ventral face of the caterpillar; Pelopæus, we believe, stabs the spider in the cephalothorax, and probably the several species of Pompilus do the same. Astata bicolor adopts the same tactics in capturing her bugs, while it is said of the fly-catchers that they commonly overcome their victims without using the sting. It is by instinct, too, that these wasps take their proper food supply, one worms, another spiders, a third flies, moths, or beetles. So strong and deeply seated is the preference, that no fly robber ever takes spiders, nor will the ravisher of the spiders change to beetles or bugs.[ill295]
PARALYZED SPIDER HUNG UP ON SORREL BY QUINQUENOTATUS WHILE SHE DIGS HER NEST
The mode of carrying their booty is a true instinct. Pompilus takes hold of her spider anywhere, but always drags it over the ground, walking backward; Oxybelus clasps her fly with the hind legs, while Bembex uses the second pair to hold hers tightly against the under side of her thorax. Each works after her own fashion, and in a way that is uniform for each species.
The capturing of the victim and caring for it before the hole is made, as in the case of P. quinquenotatus, or the reverse method, pursued by Astata, Ammophila, Bembex, and others, of preparing the nest before the food supply is secured, is certainly instinctive; as is also the way in which some of these wasps act after bringing the prey to the nest. For example, S. ichneumonea places her grasshopper just at the entrance to the excavation, and then enters to see that all is right before dragging it in. Under natural conditions this order is never varied, although the wasp can adapt herself to different circumstances when occasion demands. Again, we see Oxybelus scratching open her nest while on the wing, and entering at once with the fly held tightly in her legs. Each way is characteristic of the species, and would be an important part of any definition of the animal based upon its habits.
The general style of the nest depends upon instinct. Trypoxylon uses hollow passages in trees, posts, straws, or brick walls; Diodontus americanus, a member of the same family, always burrows in the ground, as do Bembex, Ammophila, and Sphex. In the case of Trypoxylon the passage may be ready for use or may require more or less preparation; the instinctive part is the impulse that requires the insect to use a certain kind of habitation. Any one familiar with T. rubrocinctum would never look for her nest in standing stems or under stones; to use Mr. Morgan’s test, he would be willing to bet on the general style of the dwelling-place. All of these acts are similarly performed by individuals of the same sex and race, not in circumstantial detail but quite in the same way in a broad sense. Variation is always present, but the tendency to depart from a certain type is not excessive. In Cerceris the burrow is tortuous, this style of work being common to many species in the genus, and very characteristic. No Sphex nor Ammophila constructs any such tunnel. The adherence of all the members of a species to a certain style of architecture is, then, due to instinct.
The spinning of the cocoon, in those species in which the larva is protected in this manner, and its shape, are instinctive. We find that closely allied species in the same genus make very different cocoons, as is seen in T. rubrocinctum and T. bidentatum. Some wasps spin no such covering for themselves. It is a well-known fact that silkworms sometimes omit the spinning of a cocoon; but this does not affect the argument, since the descendants of these individuals make the characteristic covering. Such cases are probably due to individual variation or perhaps to atavism, this throwing back being not uncommon among forms that are well known.
Not all of the instinctive acts here enumerated are displayed by each species studied, although they are common to most of them. We have doubtless overlooked some activities that should come under this head, as we have not made a thorough study of any sufficient number of species to make a final settlement of the matter.
As we have seen with Ammophila and Pelopæus, faults of instinct are not uncommon, but of all our wasps the one that shows the greatest aberrations is Pompilus biguttatus. The sandy beach of Lake Michigan is a favorite nesting-ground with this species, and is the scene of many a bold robbery, since they are unprincipled little wretches and