4 
Psyche 
[March 
almost say that the great efficacy of the mask has made 
unnecessary any great specialization in instinct; and, con- 
sequently, the diet of any species or individual varies 
greatly, depending upon circumstances. This statement is 
abundantly supported by observations made on the diet of 
dragonfly nymphs by several entomologists. These show 
that there is practically no selection since the imagines of 
a single species of dragonfly will devour a greatly diverse 
mixture of insects, while the nymphs consume also many 
small crustaceans, other invertebrates, and even some 
Protozoa. 
Among the dragonflies, then, a long history with com- 
paratively slow evolutionary change is associated with an 
indiscriminate diet during both the nymphal and reproduc- 
tive stages. 
If we now turn to certain predatory types among some of 
the more recent holometabolous insects we find a condition 
strikingly in contrast to that just described. In the Diptera, 
for example, there is a series of rather closely related fam- 
ilies, that includes the robber-flies or Asilidse, whose mem- 
bers are highly predaceous. Although only fragmentary 
data are available for these flies, some species are seen to 
have very strong predilections in the choice of prey. Thus, 
among the large flies of the genus Proctacanthus, one species 
captures almost entirely aculeate Hymenoptera, more than 
half of its prey consisting of honeybees and hornets, while 
the second greatly prefers small scarabseid beetles in com- 
bination with other miscellaneous insects. Other robber- 
flies are butterfly hunters, but many are more or less gen- 
eral feeders, and a tabulation shows that, although there 
is a consistent choice of food among species, there is not the 
close restriction that prevails among parasitic insects, nor 
among vegetarian types which we will discuss in a moment. 
A very high specificity in the selection of prey obtains 
among the solitary wasps. With these insects the prey is 
stung and stored in the nest, where it forms the food of the 
developing larva. Thus, the choice of food rests with the 
mother wasp, while the larva obediently eats what is put 
before it, thriving to maturity thereon. Although a very 
wide range of prey, including spiders and the most diverse 
insects is utilized by these wasps, individual species com- 
