From our general knowledge of animal nature, we should expect to find parasitic forms ready to take advantage of the material stored by such insects as the solitary wasps and the mason bees. It is said that _Chalicodoma_ provides nourishment to the larvæ of some sixteen unbidden guests. A parasitic bee (_Stelis nasuta_) breaks open a closed cell, and, after depositing its eggs, seals it up again with mortar. Since her eggs and larvæ develop more rapidly than those of the mason bee, they are first served with the store of provision, while the rightful owner is done out of its inheritance. By a curious act, of what appears to us like retributive justice, these parasitic larvæ sometimes fall a prey to another parasite, also a hymenopterous insect named _Monodontomerus_, the larvæ of which prey on the young of both bees. Another genus of the same family, _Leucopsis_ (Fig. 13, F), also succeeds in piercing with its ovipositor, at a suitable spot, the walls of the _Chalicodoma_ cell, and suspends its curious hooked egg (Fig. 13, G) on the delicate cocoon within which the chrysalis lies. Fabre found in some cases as many as five of these parasitic eggs on a single cocoon. But he never found more than one larva in any cell that he examined. The following is an epitome of his conclusions and inferences. From the parasitic egg is hatched a minute arched grub, with relatively large head and mandibles, and provided with a number of bristles, which aid it in progression (Fig. 13, H). It does not, however, at once attack the bee larva, but makes a series of excursions, the object of which is to reach and destroy any other parasitic eggs. This was not actually observed, but the eggs were found to have been destroyed, and there was seemingly no other means of destruction under the conditions maintained. The larva, this done, changes its skin and takes on a new form, destitute of bristles, with a very small head and minute mandibles (Fig. 13, I). In this new form it attacks the _Chalicodoma_ larva, making a very small incision, through which the juices of the host are transferred to the guest without further injury to the grub. It is interesting to note that, if the facts are accurately described and the inferences are correct, there are associated with two types of instinctive behaviour two distinct types of structure. The creature can have no conscious control over its structural development, and there is no ground for assuming that it has any control over its instinctive behaviour. [Illustration: FIG. 13.--Insect Larvæ. A, B, of _Sitaris_; C, D, E, of _Argyromœba_; G, H, I, of _Leucopsis_; F, imago of _Leucopsis_ (after Fabre).]