INSTINCTIVE BEHAVIOUR IN INSECTS 79 
act, of what appears to us like retributive justice, these para- 
sitic larvae sometimes fall a prey to another parasite, also a 
hymenopterous insect named Monodontomerus, the larve 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, @) on the 
delicate cocoon within which the chrysalis les. 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 con- 
clusions 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 pro- 
gression (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 destruc- 
tion 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, 1). 
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 ccntrol 
over its structural development, and there is no ground for 
assuming that it has any control over its instinctive behaviour. 
The specialization of structure and of instinctive behaviour, 
in accordance with a definite sequence of life-conditions, is 
even more remarkable in another of the many parasites which 
Chalicodoma unwittingly labours to nourish. This time it is 
a fly (Argyromeba), which lays a minute egg on the outside 
of the cell. From this egg is hatched a slender threadlike 
