108 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
The most interesting cases of the social habit are to be found 
among Insects and Backboned Animals, which it will be well to 
consider separately. 
SOCIAL INSECTS (INSECTA) 
Some of the most remarkable facts in natural history have 
been made known by those who have studied the complex com- 
munities existing among various species of Membrane- Winged 
Insects (Hymenoptera) and Net-winged Insects (Neuroptera). 
The extent to which division of labour is carried varies greatly 
in different cases, so that we are able to get some notion of the 
successive stages which have marked the evolution of the social 
habit. 
SoctaL Memprane-WINGED Insects (HyMENopTERA).— The 
most salient point distinguishing highly organized communities of 
Bees, Wasps, and Ants is the presence of a varying number of 
“castes” or kinds of individual. In the Honey-Bee (Ages medli- 
fica), for instance, a hive contains not only males (drones) and an 
egg-producing female (queen), but also numerous “ workers”, 
which are a second kind of female, having nothing to do with the 
production of eggs, but, as their name indicates, labouring for the 
benefit of the republic. And in other cases there may be more 
than three castes, as we shall see in the sequel. Worker honey- 
bees differ markedly in size and structure from the queen, as the 
result of a long course of evolution, and it will be desirable to 
begin with simpler communities, where such sharp distinctions do 
not exist. 
Bres.—Some account has already been given of Carpenter- 
Bees, Mason-Bees, and Leaf-cutting Bees, solitary forms in which 
the female not only lays eggs but also makes and stores a nest 
(see vol. iii, p. 390). These and many other non-colonial insects 
exhibit very elaborate adaptations to their surroundings, and it 
would be a mistake to consider them as necessarily lower in the 
scale than colonial forms, which have evolved on entirely different 
lines. Here, as in all other cases, success in the struggle for 
existence may be attained in widely different ways. 
From the purely solitary life led by the bees above-mentioned, 
we pass to a curious case described by Fabre (from his observa- 
tions in South France), where a certain amount of co-operation is 
