1934 ] 
Growth and Determinate Size in Insects 
43 
This determinate and definitive size involves certain gross 
characteristics, irrespective of the multiplicity of factors 
which have produced it, that are patent to any student of 
taxonomy. Those to which I shall refer to-day relate to 
comparative imaginal size in insects, although, since ab- 
solute size is directly dependent upon rate and time of 
growth, these cannot be eliminated completely from any dis- 
cussion of size. 
We usually speak of insects as small animals, but there 
exist great variations in size in practically every extensive 
taxonomic group. We find that some beetles, for example, 
are in bulk several million times as large as other beetles, 
if we compare the largest Scarabseoidea with certain minute 
Ptiliidse. Many less striking differences appear in other 
orders and I have been tempted to draw upon an admittedly 
scattered knowledge of insects in general to illustrate 
certain size peculiarities in various groups. 
Insects are by no means an exception among extensive 
groups of animals by reason of their great variation in size, 
as very similar or greater diversity may be pointed out in 
other classes of animals, as the Crustacea, fishes or mam- 
mals, or in the Phylum Mollusca, if we compare certain 
minute gastropods with giant squids. In all animals size 
shows a marked correlation with taxonomic groupings, i. e. y 
size must be regarded as having an inherently stable genetic 
basis which is its primary determinant. Secondarily it is 
modified with reference to food or climate, but such modifica- 
tions do not show any great constancy, regularity or extent. 
Thus in the tropics the size of certain types of insects is in- 
creased, for example, many scarabseoid beetles are repre- 
sented there by species much larger than those in cooler re- 
gions, and the same is true in some other groups. On the 
other hand the reverse obtains among other types, although 
it must be admitted that tropical climates on the whole 
favor larger size in insects if we compare members of the 
same group and especially species in single families. Among 
marine invertebrates large size is very often associated 
with cold waters, but a reverse condition occurs in some 
groups. The largest living terrestrial mammals are tropi- 
cal, yet in some families the species are decidedly larger in 
