In this order there cannot be the least doubt as to the
form-relationship of the imagines. The characteristic combination of
the pro- and meso-thorax, the number and venation of the wings, and the
mouth-organs formed for biting and licking, are found throughout the
whole order, and leave no doubt that the Hymenoptera are well based on
their imaginal characters.
But it is quite different with the larvæ. It may be boldly
asserted that the order would never have been founded if the larvæ
only had been known. Two distinct larval types here occur, the
one--caterpillar-like--possessing a distinct horny head provided with
the typical masticatory organs of insects, and a body having thirteen
segments, to which, in addition to a variable number of abdominal legs,
there are always attached three pairs of horny thoracic legs: the
other type is maggot-shaped, without the horny head, and is entirely
destitute of mouth-organs, or at least of the three pairs of typical
insect jaws, and is also without abdominal and thoracic legs. The
number of segments is extremely variable; the larvæ of the saw-flies
have thirteen besides the head, the maggot-shaped larvæ of bees possess
fourteen segments altogether, and the gall-flies and ichneumons only
twelve or ten. We should be much mistaken also if we expected to find
connecting characters in the internal organs. The intestine is quite
different in the two types of larvæ, the posterior opening being absent
in the maggot-like grubs; at most only the tracheal and nervous systems
show a certain agreement, but this is not complete.
The order Hymenoptera, precisely speaking and conceived only
morphologically, exists therefore but in the imagines; in the larvæ
there exist only the caterpillar- and maggot-formed groups. The former
shows a great resemblance to Lepidopterous larvæ, and in the absence
of all knowledge of the further development it might be attempted to
unite them with these into one group. The two certainly differ in
certain details of structure in the mouth-organs and in the number of
segments, abdominal legs, &c., to a sufficient extent to warrant their
being considered as two sub-orders of one larval order; but they would
in any case be regarded as much more nearly related in form than the
caterpillar- and maggot-like types of the Hymenopterous larvæ.
Is it not conceivable, however, that the imagines of the
Hymenoptera--that ichneumons and wasps may be only accidentally alike,
and that they have in fact arisen from quite distinct ancestral forms,
the one having proceeded with the Lepidopterous caterpillars from one
root, and the other with the grub-like Dipterous larvæ from another
root?
This is certainly not the case; the common characters are too
deep-seated to allow the supposition that the resemblance is here
only superficial. From the structure of the imagines alone the common
origin of all the Hymenoptera may be inferred with great probability.
This would be raised into a certainty if we could demonstrate the
phyletic development of the maggot-formed out of the caterpillar-formed
Hymenopterous larvæ by means of the ontogeny of the former. From the
beautiful investigations of Bütschli on the embryonic development of
bees[197] we know that the embryo of the grub possesses a complete
head, consisting of four segments and provided with the three typical
pairs of jaws. These head segments do not subsequently become formed
into a true horny head, but shrivel up; whilst the jaws disappear with
the exception of the first pair, which are retained in the form of soft
processes with small horny points. We know also that from the three
foremost segments of the embryo the three typical pairs of legs are
developed in the form of round buds, just as they first appear in all
insects.[198] These rudimentary limbs undergo complete degeneration
before the birth of the larva, as also do those of the whole[199] of
the remaining segments, which, even in this primitive condition, show a
small difference to the three foremost rudimentary legs.
The grub-like larvæ of the Hymenoptera have therefore descended from
forms which possessed a horny head with antennæ and three pairs
of gnathites and a 13-segmented body, of which the three foremost
segments were provided with legs differing somewhat from those of the
other segments; that is to say, they have descended from larvæ which
possessed a structure generally similar to that of the existing saw-fly
larvæ. The common derivation of all the Hymenoptera from one source is
thus established with certainty.[200]
But upon what does this great inequality in the form-relationship of
the larvæ and imagines depend? The existing maggot-like grubs are
without doubt much further removed from the active caterpillar-like
larvæ than are the saw-flies from the Aculeate Hymenoptera. Whilst
these two groups differ only through various modifications of the
typical parts (limbs, &c.), their larvæ are separable by much
deeper-seated distinctions; limbs of typical importance entirely
vanish in the one group, but in the other attain to complete
development.
In the Hymenoptera there exists therefore a very considerable
incongruence in the systems based morphologically, _i.e._ on the pure
form-relationships of the larvæ and of the imagines. The reason of this
is not difficult to find: _the conditions of life differ much less in
the case of the imagines than in that of the larvæ_. In the former the
conditions of life are similar in their broad features. Hymenoptera
live chiefly in the air and fly by day, and in their mode of obtaining
food do not present any considerable differences. Their larvæ, on the
other hand, live under almost diametrically opposite conditions. Those
of the saw-flies live after the manner of caterpillars upon or in
plants, in both cases their peculiar locomotion being adapted for the
acquisition and their masticatory organs for the reduction of food.
The larvæ of the other Hymenoptera, however, do not as a rule require
any means of locomotion for reaching nor any organs of mastication for
swallowing their food, since they are fed in cells, like the bees and
wasps, or grow up in plant galls of which they suck the juice, or are
parasitic on other insects by whose blood they are nourished. We can
readily comprehend that in the whole of this last group the legs should
disappear, that the jaws should likewise vanish or should become
diminished to one pair retained in a much reduced condition, that the
horny casing of the head, the surface of attachment of the muscles of
the jaws, should consequently be lost, and that even the segments of
the head itself should become more or less shrivelled up as the organs
of sense therein located became suppressed.
The incongruence manifests itself however in yet another manner than
by the relatively greater morphological divergence of the larvæ: a
different grouping is possible for the larvæ and for the imagines. If
we divide the Hymenoptera simply according to the form-relationships of
the imagines, the old division into the two sub-orders _Terebrantia_
or _Ditrocha_ and _Aculeata_ or _Monotrocha_ will be the most correct.
The distinguishing characters of a sting or ovipositor and a one- or
two-jointed trochanter are still of the greatest value. But these
two sub-orders do not by any means correspond with the two types of
larvæ since, in the _Terebrantia_, there occur families with both
caterpillar-formed and maggot-formed larvæ.
The cause is to be found in that a portion of these families possess
larvæ which are parasitic in other insects or in galls, their bodily
structure having by these means become transformed in a quite different
direction. The mode of life of the imagines is, on the other hand,
essentially the same.
We have here therefore another case like that which we met with among
the Rhopalocerous Lepidoptera, in which the imagines appear to be
capable of being formed into a higher group than the larvæ, because the
former live under conditions of life which are on the whole similar
whilst the latter live under very divergent conditions.
The old division of the Hymenoptera into two sub-orders has certainly
been abandoned in the later zoological text-books; they are now divided
into three:--saw-flies, parasitic, and aculeate Hymenoptera; but even
this arrangement has been adopted with reference to the different
structure of the larvæ. Whether this system is better than the older,
_i.e._ whether it better expresses the _genealogical_ relationship, I
will not now stop to investigate.[201]