
694 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
larval integument. Similarly, it is apparent that the imaginal 
-antennze and compound eyes are in the one case evaginations and 
in the other simply modified portions of this imaginal derm ; 
and although I have not made cuttings of a complete series 
of heads from young to oldest larvae, enough of the younger 
stages have been studied to show the simple dermal origin of 
al these parts by a continuous process of evagination and . 
modification. We are sufficiently acquainted with the origin 
and mode of development of the legs and wings of insects 
from histoblasts to recognize in these histoblasts, or develop- 
mental centers, simple invaginations of the derm, which later 
become evaginations. Whether an organ, as wing, leg, antenna, 
or mouth part, shall begin as an invagination or an evagination 
of the derm is chiefly a matter of mechanical necessity or ease, 
and of degree of radicalness in the metamorphosis. In either 
case the ultimate origin, that of being simply a particular 
portion or area of derm, is the same; the invagination must 
become an evagination; the difference lies in the mechanical 
factors of the developmental process. 
HYMENOPTERA. 
In the order Hymenoptera there is to be found, as in the 
Lepidoptera, a wide range of degree of specialization of the 
mouth parts, varying from the biting, orthopterous mouth of 
the sawflies to the highly modified sucking mouth of the honey- 
bee ; but throughout the order the mandibles persist in plainly 
jawlike character, and are always recognizable landmarks in 
mouth-part dissections. The only questions in the homology- 
interpretation occur in those cases where the labium and 
maxillae are much modified and more or less completely fused 
or bound together. But these questions are not very serious; 
entomologists are fairly agreed, on a basis of comparative 
anatomical study, on the interpretation of the homologies of 
the hymenopterous mouth parts. But the results of a study of 
the post-embryonic development of the mouth parts, 7.¢., the 
development of the imaginal mouth parts, undertaken by one 
of my students, Mr. M. H. Spaulding, illuminate too beautifully 
