PSYCHE. 
maxilla and the under surface of the head 
(fig. 2, 7) has become a clearly defined 
line, and the whole supporting framework 
of the labium and maxillae approaches a 
rectangular form, limited posteriorly by 
chitinous supports (fig. 4,2, h’). Muscles 
extend from these chitinous supports, h, h’, 
anteriorly to the chitinous rod / and pos- 
teriorly to the rod g. The action of these 
muscles protrudes or retracts the framework 
of the labium and maxillae, but, while the 
maxillae are simply pushed forward by 
this action, the labium is unfolded, or un- 
rolled, so as to assume the position shown 
in fig. 3, v’. 
If the form of the trophiin Gracilaria 
syringella, in its early larval stages, be 
compared with the form in the later larval 
stages, the modification of each part will 
be seen to increase in amount as the dis- 
tance from the anterior part of the head 
increases, that is, the mandibles change 
more than the labrum, the maxillae more 
than the mandibles, and the labium most 
of all. By comparing, on fig. 3, the mouth- 
parts of the early and later forms, the 
changes of the labrum are seen to be 
mostly in markings, and in the number of 
hairs or spines; the changes of the man- 
dible have been, to a slight extent, modifi- 
cations of form; the maxillae, while they 
existed before, are greatly modified in the 
later form; the labium most of all seems 
to have added to its structure and functions. 
This increase of complexity of the mouth- 
parts toward the labium, which becomes 
more and more striking at each successive 
stage in many insects, is probably coordin- 
ate with the relations which these parts 
have to the surfaces on which the larvae 
103 
walk, from which they feed, and, in many 
cases, on which they spin their silk. 
This successive development of organs 
coordinate with their functions is well 
exemplified in the larvae of Gracilaria, of 
Inthocolletis, of Phyllocnistis * and possibly 
of other Tineina, in which the early form 
of the larva is so different from its later 
form. In Gracilaria syringella, so long 
as the larvae feed only in a plane surface, 
and do not spin, no great difference is visi- 
ble between the mouth-parts above the 
mandibles and those below them. Both 
labrum and labium exercise their simplest 
functions of pushing the food between, and 
keeping it subject to the action of the man- 
dibles, and, perhaps, of sawing it out of 
the parenchyma by a lateral motion. But 
contemporaneous with the appearance of 
the later form of larval trophi, — the la- 
bium capable of turning under, and the 
better developed maxillae — the larva, now 
having its mandibles directed slightly ven- 
trally, acquires the power of eating verti- 
cally to the surface of the leaf, and of 
spinning threads in all directions. At the 
same time its feet are developed, later it 
eats out from its mine, and becomes free. 
An externally well-differentiated dorsal and 
ventral surface seems to have been acquired 
from the time when the larva ceased to be 
pressed dorsally and ventrally between the 
upper and lower epidermis of its mine. 
I have not had the opportunity to com- 
pare, by microscopic work, the larva of 
Gracilaria syringella with other larvae of 
Tineina; therefore the above observations 
may be regarded as quite incomplete. 
Leipzig, 15 Aug. 1880. 
8PsycuE, May [July] 1880, v. 3, p. 67. 
