1934] Growth and Determinate Size in Insects 47 
respect to size. Speaking in very general terms, several 
orders of insects include consistently small or minute 
species, for example the Protura, Collembola, Thysanoptera, 
Zoraptera, Mallophaga, Anoplura, Siphonaptera, Strepsip- 
tera and Corrodentia, the last group being less appropri- 
ately cited as it includes a few moderate-sized species. 
Other orders include entirely or in great part large species, 
such as the Phasmatodea, Mantodea, Orthoptera, Blattarise, 
Odonata and Megaloptera. It is noticeable in the latter 
series that small forms do occur here and there in practi- 
cally all of the orders cited as there are a few very small 
Blattarise and Orthoptera; moreover some Mantodea and 
Odonata are by no means large insects. In some orders 
size is extremely variable, for example in the Hemiptera, 
Neuroptera, Trichoptera, Lepidoptera, Diptera, Hymenop- 
tera and Coleoptera. By far the majority of insects are in- 
cluded in these seven orders of which the last four are far 
and away the most extensive. 
Several matters relating to size stand out very clearly 
from the rough approximations just outlined. Extensive 
groups composed of large species produce occasionally or 
sporadically small or dwarf types. Thus it appears that a 
reduction in size has been accomplished far more easily dur- 
ing the course of evolution in insects than has the reverse 
change whereby small types have given rise to large ones. 
The most extensive orders where speciation has been most 
active and where the appearance of diverse types has pro- 
ceeded at a rapid rate, exhibit the widest range in size. It 
follows therefore that size and form show a strong tendency 
to remain constant or to vary together, not to be modified 
independently of one another. This is strong evidence that 
size has a strongly fixed genetic basis since it does not 
readily change, except in groups where morphological di- 
versification and adaptation has been most extensive. 
Even in these orders the fixity of size is well illustrated 
if we compare the subdivisions such as superfamilies or 
families. Among the Hymenoptera, for example, certain 
superfamilies show great constancy in size. The super- 
families Serphoidea, Chalcidoidea, and Cynipoidea include 
only small or minute species, with really only a single com- 
