32 INSTINCTIVE BEHAVIOUR 
Thousands must go astray. They have been found on hairy 
beetles, flies, and bees of the wrong genus. Some, however, 
become thus attached to the one suitable species, and are con- 
veyed by the Anthophora to her nest, where they promptly eat 
the egg she lays. It is not difficult to picture to one’s self how 
this incompletely evolved instinct might be further perfected 
by natural selection, through the survival of those females 
which laid their eggs in the haunts of the bee-host. And 
such an advance in instinctive behaviour is seen in another 
and rarer beetle—Sitaris. Her eggs are laid in August near 
the entrance to a nest of the Anthophora. In September they 
hatch to form larvee, which hibernate in groups till the following 
spring. Then they become active (Fig. 13, A), and attach 
themselves to hairy objects. Being near the Anthophora nest, 
there is an increased chance of their fastening upon this bee. 
The chance is still far from good, for if this were so, we should 
not find that the Sifaris laid as many as two thousand eggs. 
Still, on these grounds, we may presume that its chance of 
survival is about five times as good as that of J/e/oé, which 
lays ten thousand eggs. The larva is said generally to attach 
itself to a male bee, which is hatched earlier than his mate, 
and to pass on to the female at the nuptial period ; but in any 
case it eventually slips on to the egg that she lays. This 
forms the food of the larva during the remainder of this stage 
of its existence. It then moults and assumes a new form, 
capable of feeding on the honey (Fig. 13, B) ; and, after further 
changes, becomes a pupa, and then assumes the imago condition. 
In these cases the advantage is wholly on the side of the 
parasite. But there are cases of close relationship between 
insects and flowering plants where the instinctive behaviour 
gives rise to reciprocal benefit. The Yucca is a genus of 
American Liliaceous plants, with large pale sweet-smelling 
flowers; and these are dependent for fertilization on the 
instinctive behaviour of a small straw-coloured moth of the 
genus Pronuba. Just when the Yucca plant blossoms in 
the summer, the moths emerge from their chrysalis cases. 
They mate; and the female then flies to a flower, collects a 
