
684 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
These homology-determinations were first made by a study 
of the comparative anatomy of the fully developed mouth parts 
(those of adult insects), and indeed have a fairly safe ground- 
ing on this comparative anatomical study alone. But with the 
development of embryological studies of insects came the con- 
firmation of these determinations, or some of them, by the study 
of the development of the mouth parts. From their origin as 
budding appendages, arising on the successive segments of the 
embryonic head, their development has been readily and cer- 
tainly traced to the definitive mouth-part condition ; and man- 
dibles, maxillae, and labium are as certainly serially homologous 
with each other and with the legs and antennz as are the more 
obviously homologous segmental appendages of the crustaceans. 
But this ontogenic development of the insectean mouth 
parts, simple and continuous as it is in the case of insects with 
an incomplete metamorphosis, is a very complex and difficult 
subject of study in all of those insects which undergo what is 
termed a complete metamorphosis, and this for the reason, 
now familiar to entomologists, that in the late larval and early 
pupal life of such insects a more or less radical histolysis, or 
breaking down of the larval organs and tissues, occurs, with a 
building up of the imaginal organs from small, primitive cen- 
ters called histoblasts (imaginal disks), which are not derived 
from the corresponding larval organs but (for the appendages 
as legs and mouth parts) from the larval derm or cellular skin 
layer. Thus we have in the development of the mouth parts 
of insects with complete metamorphosis a discontinuity which 
sadly interferes with the determination of homologies by 
ontogenetic study. Indeed, so serious has this obstacle proved 
that we have as yet practically no complete tracing through 
both embryonic and post-embryonic development of the 
growth and development of the mouth parts of any insect of 
complete metamorphosis. And they are, for the most part, 
precisely those insects of most radical post-embryonic meta- 
morphosis which possess in adult condition the most highly 
modified and specialized mouth parts, and which present to us the 
most serious task in the interpretation of the mouth-part homol- 
ogies. The Diptera, of course, best exemplify these conditions. 
