the _Bembex_, which especially attacks Diptera to make them the
prey of its larvae, throws itself suddenly on them and kills them with
one blow in any part of the body. It is unable in this way to amass in
advance sufficient provision for its larvae; the corpses would putrefy.
It is obliged to return from time to time bearing new pasture.[9]
Again, M. Paul Marchal, taking up the study of instinct in the
_Cerceris ornata_,[10] has shown that in this species at least of
_Sphegidae_ the stings have not so considerable an effect. This insect
attacks a wild bee, the _Halictus_. He strikes his victim with two or
three strokes of the sting beneath the thorax, but the paralysis is
not definite, perhaps on account of the nature of the venom, which is
not identical in all species. The tortured creature may regain life at
the end of some hours. Thus the _Cerceris_ is obliged to destroy the
upper part of the neck by repeated malaxation of that part for several
minutes at a time. The effect of this second act, by injuring the
cerebroid ganglia, is to render impossible the return of action;
moreover, it permits the aggressor to satisfy personal gluttony, and
to feed on the liquids of the organism of the vanquished, which is
easy, because the dorsal blood-vessel passes at this level. It can
thus satisfy a personal need while thinking of the future of the race.

[9] J. H. Fabre, _Souvenirs entomologiques_, pp. 225 _et seq._

[10] "Etude sur l'Instinct du _Cerceris ornata_," _Archives
de Zoologie experimentale_, ii. Serie, t. 5, 1887.

It has been said in this connection that in such cases the sure
instinct with which these species were originally endowed has been
distorted, but that is to admit some degree of variation; the
hypothesis of degeneration is as gratuitous as the other, and if we go
so far as to risk a hypothesis, it would be better to use it to
explain facts and not to entangle them.

_Plan of study of the various industries._--The different industries
carried on by animals may be divided into a certain number of groups.
In the case of each of these categories I propose to arrange the facts
in such a way as to bring forward first those animals which, having no
special organs, are obliged to exercise the greatest ingenuity, and
then to indicate the facts which show how variations have arisen which
enable other species to accomplish these acts with marvellous ease.

We will first examine the simplest indust