144 
which made a suctorial mouth advantageous, those indi- 
viduals would be favoured by natural selection in which 
the mandibles and maxilla were best calculated to pierce 
or prick, and their power of lateral motion would tend to 
fall into abeyance ; while, on the other hand, if powerful 
masticatory Jaws were an advantage, the opposite process 
would take place, 
There is yet a third possibility—namely, that during 
the first portion of life, the power of mastication should 
be an advantage, and during the second that of suction, or 
vice versé. A certain kind of food might abound at one 
season and fail at another, might be suitable for the animal 
at one age and not at another : now in such cases we should 
have two forces acting successively on each individual, 
and tending to modify the organisation of the mouth in 
different directions. It will not be denied that the ten 
thousand variations in the mouth-parts of insects have 
special reference to the mode of life, and are of some ad- 
vantage to the species in which they occur. Hence no 
believer in natural selection can doubt the possibility of 
the three cases above suggested, the last of which seems 
to explain the possible origin of species which are man- 
dibulate in one period of life and not in another. The 
change from the one condition to the other would no doubt 
take place contemporaneously with a change of skin. At 
such times we know that, even when there is no change 
to form, the temporary softness of the organs precludes 
the insect from feeding for a time, as, for instance, 
is the case with the silkworm. When, however, any con- 
siderable change was involved, this period of fastinz 
would be prolonged, and would lead to the existence of a 
third condition, that of the pupa, intermediate between 
the other two, Since other changes are more conspicuous 
than those relating to the mouth, we are apt to associate 
the existence of a pupa-state with the acquisition of 
wings: but the case of the Orthoptera (grasshoppers, 
&c.) is sufficient proof that the development of wings 
is perfectly compatible with continuous activity ; so that 
in reality the necessity for rest is much more intimately 
connected with the change in the constitution of the 
mouth, although in many cases no doubt the result is 
accompanied by changes in the legs, and in the in- 
ternal organisation. An originally mandibulate mouth, 
however, like that of a beetle, could not, I think, be 
modified into a suctorial organ like that of a bug or 
a gnat, because the intermediate stages would necessarily 
be injurious. Neither, on the other hand, for the same 
reasons, could the mouth of the Hemiptera be modi- 
fied into a mandibulate type like that of the Coleop- 
tera. But in Camfodea and the Collembola we have a 
type of animal closely resembling certain larve which 
occur both in the mandibulate and suctorial series of in- 
sects, and possessing a mouth neither distinctly mandibu- 
late, nor distinctly suctorial, but constituted on a peculiar 
type capable of modincation in either direction by gradual 
change, without loss of utility. 
In discussing this subject it is necessary also to 
take into consideration the nature and origin of wings. 
Whence are they derived? why are there normally two 
pairs ? and why are they attached to the meso- and meta- 
thorax? These questions are not less difficult than inte- 
resting. It has been suggested, and I think with justice, 
that the wings of insects originally served for aquatic and 
respiratory purposes. From the various modes by which 
respiration is effected among the different groups of aquatic 
insects, there are strong reasons for concluding that the 
original insect stock was, like Campoaea (P13, Fig. 5), a 
land animal. But in aquatic insects there is a tendency to 
effect the purification of the air through the delicate mem- 
braneous covering of more or less leaf-like expansions 
of the skin, In the larva of Chloéon (Pl. 4, Fig. 1), for 
instance, that singularly resembles Camoaea (Pl. 3, Fig. 
5), several of the segments are provided with such folia- 
ceous expansions ; which, moreover, are in constant agi- 
NATURE 
[Fune 19, 1873 
tation, the muscles of which, in several remarkable points, 
resemble those of the true wings. It is true that in 
Chloéon the vibration of the so-called branchiz is scarcely, 
if at all, utilised for tke purpose of locomotion ; the 
branchiz are, in fact, placed too far back to act effi- 
ciently. The situation of these branchiz differs in dif- 
ferent groups ; indeed, it seems probable that originally 
there were a pair on each segment. In such a case, 
those branchiz, situated near the centre of the body, 
neither too much in front nor too far back, would serve 
the most efficiently as propellers. The same causes 
which determined the position of the legs would affect 
the wings also, Thus a division of labour would be 
effected ; the branchiz on the posterior segments of 
the thorax would be devoted to locomotion; those on the 
abdomen to respiration. This would tend to increase the 
development of the thoracic segments, already somewhat 
enlarged to receive the muscles of the legs, 
That wings may be of use to insects under water is 
proved by the very interesting case of Polynema natans, 
which I discovered in 1862, and which uses its wings to 
swim with. This, however, is a rare case; and it is 
possible that the principal use of the wings was, primor- 
dially, to enable the mature forms to pass from pond 
to pond, thus securing fresh habitats and avoiding in-and- 
in breeding. If so, the development of wings would 
tend to be relegated to a late period of life; and by 
the tendency to the inheritance of characters at corre- 
sponding ages,to which Mr. Darwin has called attention,* 
the development of wings would be associated with the 
maturity of the insect. Thus the late acquisition of 
wings in the Insecta generally, seems to be itself an indi- 
cation of their descent from a stock which was at one 
period aquatic in its habits, and which probably re- 
sembled the present larvae of Ch/oéon in form, but had 
thoracic as well as abdominal branchiz. 
If these views are correct, the genus Campodea must 
be regarded as a form of remarkable interest, since it is 
the living representative of a primzeval type from which 
not only the Collembola and Thysanura, but the other 
great orders of insects have all derived their origin. 
Finally, from the subject of metamorphoses we pass 
naturally to that most remarkable phenomenon which is 
known as the “ Alternation of Generations :” for the first 
systematic view of which we are indebted to my eminent 
friend Prof. Steenstrup. 
I have always felt it very difficult to understand why 
any species should have been created in this double 
character ; nor, so far as I am aware, has any explanation 
of the fact yet been attempted. Yet insects offer, in the 
metamorphoses which they go through, a phenomenon 
not altogether dissimilar, and give a clue to the manner 
in which alternation of generations may have originated. 
The caterpillar owes its difference from the butterfly 
to the early stage at which it leaves the egg; but its 
actual form is mainly due to the influence of the condi- 
tions in which it lives. If the caterpillar, instead of 
changing into one butterfly, produced several butterflies, 
we should have an instance of alternation of generations. 
Until lately, however, we knew of no such case; each 
larva produced one imago, and that not by generation 
but by development. It has long been known, indeed, 
that there are some species in which certain individuals 
remain always apterous, while others acquire wings. 
Many entomologists, however, regard these abnormal 
individuals as perfect, though wingless insects; and 
therefore, though these cases appear to me to deserve 
more attention than they have yet received, I shall not 
found any argument on them. 
Recently, however, Prof. Wagner of Kazan, has dis- 
covered that, among certain small gnats, the larve do 
not themselves directly produce the perfect insect, but 
give rise to other larvae, which undergo metamorphoses of 
* Origin of Species, 4th ed. pp. 14 and 97. 
