he Si 
| May 8, 1873] 
an a  &, > ) 
who had collected some of the transparent ova of Rhyz- 
chites betuleti and to his great surprise found more 
than half of them attacked by a small parasite, which 
are to be the larva of a minute Hymenopterous insect 
elonging to the Pteromalide. Fig. 15 shows the egg of 
Rhynchites, with the parasitic larva, which is represented 
on a larger scale in Fig. 16. Recently, however, this group 
has been more completely studied by M. Ganin,* who thus 
describes the development of Platygaster. The egg, asin 
other allied hymenopterous families, for instance in 
Cynips, is elongated and club-shaped (Fig. 17). After a 
while a large nucleated cell appears in the centre (Fig. 18); 
this is anew formation not derived from the germinal 
vesicle. This nucleated cell divides (Fig. 19) and sub- 
divides. The outermost cells continue the same process, 
thus forming an outer investing layer. The central one, 
on the contrary, enlarges considerably, and develops 
within itself a number of daughter cells (Figs. 20and 21), 
which gradually form themselves into a mulberry-like 
mass, thus giving rise to the embryo (Fig. 22). 
Ganin met with these larve in those of a small gnat, 
Cecidomyia. Sometimes as many as fifteen parasites oc- 
curred in one host, but as a rule only one attained maturity. 
The three species of Platygaster differed considerably 
in form, as shown in the three following Figs. (23-25). 
They creep about in the egg by means of the strong 
hooked feet, 47, somewhat aided by movements of the 
tail. They possess a mouth, stomach, and muscles, but 
the nervous, vascular, and respiratory systems do not 
make their appearance until later. After some time the 
larva changes its skin and assumes the form represented 
in Fig. 26. In this moult the last abdominal segment of 
the first larva is entirely thrown off: not merely the outer 
skin as in the case of the other segments, but also the 
hypodermis and the muscles. This larva, as will be seen 
by the figure, is in the form of a barrel or egg, and 
*870 mm. in length, the external appendages having dis- 
appeared, and the segments being indicated only by the 
arrangement of the muscles; s/4/is the cesophagus lead- 
ing into a wide stomach which occupies nearly the whole 
body, sz sae is the rudiment of the supracesophagal 
ganglia, 6s” the ventra nervous cords. The ventral 
nervous mass has the form of a broad band, with 
straight sides; it consists of embryonal cells, ani re- 
mains in this undeveloped condition, during the whole 
larval state. 
At the next moult the larva enters its third state, which, 
however, as far as the external form (Fig. 27) is con- 
cerned, differs from the second only in being somewhat 
more elongated. The internal organs, however, are much 
more complex and complete. The trachee have made 
their appearance, and the mouth is provided with a pair 
of mandibles. From this point the metamorphoses of 
Platygaster do not appear to differ materially from those 
of other Hymenoptera. 
An allied genus, Polynema, has also very curious 
larve. The perfect insect is aquatic in its habits, 
swimming by means of its wings; flying, if we may 
say so, under water. It lays its eggs inside those of 
Dragon flies ; and the larva, as shown in Fig. 28, leaves 
the egg in the form of a bottled-shaped mass of undiffe- 
rentiated embryonal cells, covered by a thin cuticle, but 
without any trace of further organisation. Protected by 
the egg shell of the Dragon fly, the young Polynema is 
early able to dispense with its own ; and bathed in the 
nourishing fluid of the Dragon fly’s egg, it imbibes 
nourishment through its whole surface, and increases 
rapidly in size. The digestive canal gradually makes its 
appearance, the cellular mass forms beneath the original 
cuticle a new skin, distinctly divided into segments, and 
provided with certain appendages. After a while the old 
cuticle is thrown off, and the larva gradually assumes the 
form shown in Fig. 29. asch are the antennal discs, or 
* Zeits. f. Wiss. Zool., 1869, 
NATURE 33 
rudiments of the antenna, /7sch of the wings, dsch of the 
legs, vfg are lateral projections, gsch of the ovipositor, 
&c., ff is the fatty tissue. The subsequent metamor- 
phoses of Polynema offer no special peculiarities. 
From these facts—and, if necessary, many more of the 
same nature might have been brought forward—it seems 
to me evident that while the form of any given larva de- 
pends to a certain extent on the group of insects to which 
it beiongs, it is also greatly influenced by the external 
conditions to which the animal is subjected ; that it is a 
function of the life which the larva leads and of the group 
to which it belongs. 
The larvee of insects are generally regarded as being 
nothing more than immature states—as stages in the de- 
velopment of the egg into the imago ; and this might 
more especially appear to be the case with those insects 
in which the larve offer a general resemblance in form 
and structure (excepting of course so far as relates to the 
wings) to the perfect insects. Nevertheless we see that 
this would be a very incomplete view of the case The 
larva and pupa undergo changes which have no relation 
to the form which they will ultimately assume. With a 
general tendency, as regards size and the production of 
wings, to this goal, there are combined other changes 
bearing reference only to their existing wants and condi- 
tion. Nor is there in this, I think, anything which need 
Fic. 28, Embryo of Polynema (after Ganin). 20, Larva of Polynema, 
asch, rudiments of the antenna; f/sch of the wings; dsch of 
the eggs ; v_/g, lateral projections ; gsc, rudiments of the ovipositor ; 
fk, tatty tissue. 
surprise us. External circumstances act on the insect in 
its preparatory states, as well as in its perfect condition. 
Those who believe that animals are susceptible of great, 
though gradual, change through the influence of external 
conditions, whether acting, as Mr. Darwin has suggested, 
through natural selection, or in any other manner, will 
see no reason why these changes should be confined to 
the mature animal. And it is evident that creatures 
which, like the majority of insects, live during different 
parts of their existence in very different circumstances, 
may undergo considerable changes in their larval orga- 
nisation, in consequence of forces acting on their larval 
‘condition ; not, indeed, without affecting, but certainly 
without affecting to any corresponding extent, their ulti- 
mate form. 
I conclude, therefore, that the form of the Jarva in 
insects, whenever it departs from th2 orizinal vermiform— 
or the later Campodea—type, depends in great measure 
on the conditions in which it lives. The external forces 
acting upon it are different from those which affect the 
mature form; and thus changes are produced in the 
young, which have reference to its immediate wants, 
rather than to its final form. 
And, lastly, as a consequence, that metamorphoses may 
be divided into two kinds, developmental and adaptional. 
