multivalve ovipositor[1191].
6. HYMENOPTERA[1192] (_Piezata_ F.). Mr. MacLeay considers _Sirex_
L. as being osculant between the Order we are now entering upon and
the _Trichoptera_, and _Tenthredo_, L. as belonging to the latter.
He appears to ground this opinion chiefly upon a consideration of
their larvæ and a slight difference in their ovipositor. As the
Order, as settled by Linné, has always been deemed one of the most
natural ones, and all the great Entomologists of the present æra have
agreed with him in thinking it so; it seems to me that to prove them
mistaken in this opinion, the question should have been discussed
at more length, and that it requires arguments of more weight than
any Mr. MacLeay has at present produced to set it aside. He appears
in general to lay great stress upon an agreement in larvæ and the
kind of metamorphosis; and I am ready to acknowledge that it forms a
strong _presumption_ in favour of any hypothesis of affinity between
certain tribes. But when it is had recourse to as fundamental and
infallible, I think it is pushed far beyond what it will bear, or
is warrantable. I may be wrong; but in my apprehension, a striking
agreement in their general structure in the _perfect_ state, which is
the acme of their nature, affords a much more satisfactory reason for
keeping two tribes together, than any difference observable in their
larvæ or metamorphosis, for separating them. Let any one compare the
structure of these two tribes with the _Trichoptera_ on one side, and
the _Hymenoptera_ on the other, and it will require but a glance to
convince him of their greater affinity to the latter; and the simple
inspection only of Jurine's plates of the wings of _Hymenoptera_ is
calculated to produce the same effect. With regard to their _larvæ_,
the resemblance between the case-worms and the pseudo-caterpillars
of the saw-flies seems to me very distant, and the numerous prolegs
of the latter have scarcely a legitimate representative in the
former. The larvæ of the genus _Lyda_ lose the prolegs intirely,
and in one species, which much resembles the vermiform larvæ of
_Hymenoptera_, the real legs are so extremely short as to be scarcely
discernible[1193]; so that it requires no great stretch of faith to
believe that saw-flies or _Sirices_ may exist in whose larvæ the legs
disappear[1194]. But it is this very tribe, whose larvæ thus approach
to those of the other _Hymenoptera_, in which Mr. MacLeay finds the
greatest external resemblance to the _Trichoptera_[1195]. In fact the
difference between the saw-flies and _Siricidæ_, and the remainder
of the _Hymenoptera_, amounts to little more than what takes place
in the _Diptera_ Order between the _Tipulidæ_, _Asilidæ_, _Muscidæ_,
&c., in which also the _metamorphosis_ differs.
Another argument upon which Mr. MacLeay seems to lay some stress,
is taken from the number of parts into which the _ovipositor_ of
the saw-flies is resolvable, which he finds to consist of _four_
pieces; while in what he considers as the genuine _Hymenoptera_, it
is formed only of _three_[1196]: but in fact, in these last there
are _two_ spiculæ, answering to the two saws of _Tenthredo_, so that
the vagina in which these move may be considered as a _double_
sheath: only, as these were to be pushed out at the _same_ time,
and the others _alternately_, it was necessary that in the latter
each sheath should be separate, to admit of this motion; but as to
its composition, the weapon in both is essentially the same. At any
rate this structure could furnish a reason only for the formation of
a separate group in the _same_ Order, but none for the transfer of
such group to _another_, which had no such instrument at all; since,
as we have seen, the _Trichoptera_ extrude their eggs at once in a
mass[1197]. I do not mean, however, that it should be inferred from
what I have here said, that there is no _tendency_ in the saw-flies
towards a Trichopterous type, for in them nature seems pointing that
way, but the distance is too great, and the number of types of form
necessary to fill up the interval too many, to warrant in my opinion
their removal from the one Order to the other.
DEF. _Metamorphosis_ incomplete[1198].
_Trophi_ in most not used for mastication[1199].
_Wings_ four: _neuration_ generally areolate[1200].
_Prothorax_ obsolete, giving place to an ample collar.
_Tarsi_ pentamerous.
_Ovipositor_ 5-6-valved, the vagina darting forth two
retroserrulate spiculæ.
** _ORDERS in which all the ordinary Trophi do_ not _occur,
or the_ Mouth _is_ imperfect[1201] (_Haustellata_).
7. HEMIPTERA[1202] (_Ryngota_ F.). Linné at first confined this Order
to those insects which have a _promuscis_, which he denominated a
_rostrum_[1203]; but afterwards, convinced that the _Orthoptera_ of
the moderns could not be associated properly with the _Coleoptera_;
instead of forming them into a distinct Order, as nature would
have dictated--perhaps to avoid the multiplication of Orders and
without altering his definition--with equal infelicity he added
them to this. Subsequent Entomologists, who saw the impropriety of
masticating insects thus herding with suctorious ones, restricted the
Order to its old limits; but Latreille very judiciously altered its
arrangement, and divided it into two Sections, separating those whose
hemelytra terminate in membrane, from those in which they are mostly
tegmina, or of a substance intermediate between that of the elytra
of _Coleoptera_ and that of the wings of the Tetrapterous Orders.
He denominated the first of these sections, or rather suborders,
_Heteroptera_, and the last _Homoptera_[1204]. Dr. Leach, observing
that very considerable differences take place both in the economy
and structure of Heteropterous and Homopterous insects, followed De
Geer in considering them as separate Orders, which he has called
_Hemiptera_ and _Omoptera_, and in which he has been followed by
Mr. MacLeay; who, however, with his usual accuracy and judgment,
has restored the aspirate to the latter name[1205]. Their agreement
in having a _promuscis_, or instrument of suction, with a jointed
sheath, at present induces me to hesitate as to the propriety of
their separation, and to consider them as forming _secondary_ rather
than _primary_ sections of the Class. That you may be enabled to
judge for yourself upon this subject, I will state the principal
features in which they differ. In the first place, the Heteropterous
section usually sucks the juices of _animals_, and the Homopterous,
those of _plants_; in the former, the _Hemelytra_, besides their
different substance, as well as the wings, cross each other; while
in the latter, the organs of flight are deflexed, and do not lap
over each other at all. The antennæ also of the one are often long,
and do not terminate in a _bristle_; while in the other, with few
exceptions, they are very short and setigerous. In the _Heteroptera_
the body is depressed and flat, in the _Homoptera_ convex and thick.
In the former, the scutellum is one of the principal features of the
trunk; in the latter, not at all remarkable[1206]. Other differences
in the structure, both of head, trunk, and abdomen, might be pointed
out; but these you will chiefly find noticed in my letters on the
External Anatomy of Insects, where I treated of those parts. I shall
here, therefore, only further mention the ovipositor also as forming
a most striking distinction[1207].
DEF. _Metamorphosis_ semicomplete in almost all.
_Mouth_ promuscidate[1208].
_Wings_ covered by _Hemelytra_ or _Tegmina_[1209].
_Tarsi_ mostly trimerous, rarely dimerous or monomerous[1210].
8. TRICHOPTERA[1211] Kirby (_Synistata_ F. _Neuroptera_ Latr.).
MM. Latreille and MacLeay are of opinion that _Semblis_ F. and
_Phryganea_ L. ought to be associated in the _same_ group; and the
latter gentleman has backed his opinion by some apparently cogent
arguments[1212]: there are others, however, that seem to me more
cogent, for considering them as belonging to _different_ Orders.
Whoever examines the several tribes into which Mr. MacLeay has
divided the _Neuroptera_, will observe in all of them a distinct
_prothorax_, a circumstance which they possess in common with those
Orders that use their mandibles for _mastication_; whereas in those
that do _not_ use them for mastication, as the _Hymenoptera_, or
that take their food by suction, this part is replaced by a mostly
narrow collar, forming a part of the alitrunk[1213]. The existence
then of the _prothorax_ in the _Perlidæ_, and of the _collar_ in
the _Trichoptera_, affords no slight presumptive evidence that
they belong to different Orders. Another circumstance that weighs
much with me is, that the type of the neuration of the wings
in _Perla_ is taken from the _Neuroptera_, in the _Trichoptera_
from the _Lepidoptera_; the same observation extends to the legs
of both[1214], and likewise to the abdomen. Even in their oral
organs, as far at least as relates to their mandibles, those of
_Perla_, though membranaceous--a circumstance occurring even
in _Coleoptera_--are of a Neuropterous type; while the angular
termination of the cheeks in the _Phryganeæ_ approaches to the
Lepidopterous mandibular rudiments. The principal argument on which
Mr. MacLeay's opinion seems to rest, is, that the larvæ of both are
aquatic, and clothe themselves in cases formed of various materials:
but though this circumstance shows that they approximate in the
system, it does not prove that they belong to the same order, since
the general habit and appearance of the two animals when arrived at
perfection contravenes it. The larvæ of _Myrmeleon_ and of _Leptis
Vermileo_ form pitfalls of sand for their prey, and when they become
pupæ, cover themselves with it[1215]; but this in them does not even
prove an affinity, but only an analogy. The larva of _Perla_ is
carnivorous[1216], that of _Phryganea_ mostly herbivorous[1217]: so
that they are not precisely similar in their habits. Whether they
resemble each other _altogether_, in their form, does not clearly
appear. The above reasons will, I trust, justify me for considering
them _at present_ as belonging to different Orders; but if further
discoveries should confirm the opinion Mr. MacLeay espouses, I shall
have no hesitation in yielding to it.
DEF. _Metamorphosis_ incomplete[1218].
_Mouth_ emandibulate.
_Prothorax_ replaced by a collar.
_Wings_ four, upper pair mostly hairy, lower ample, folded:
_neuration_ branching.
_Anus_ without setæ. _Eggs_ extruded in a gelatinous
mass[1219].
9. LEPIDOPTERA[1220] (_Glossata_ F.). Concerning this Order, no
difference of opinion exists amongst Entomologists. Besides the
scales that cover their wings, they are distinguished by the
peculiar instrument of suction formerly described: neither of these
characters, however, is perfectly universal; some of the Order
(_Nudaria_) having no scales upon their wings, and others being
without any _antlia_ (_Aglossa_). Other peculiar characters are to be
found in them; for instance, the _patagia_, or tippets, that adorn
their evanescent thorax[1221], and the _tegulæ_, or base-covers, of
a shape quite dissimilar to those of _Hymenoptera_, which cover and
defend the base of their wings[1222]. As in the last Order, their
legs are located all together with scarcely any space intervening
between them; and they often agree also in their spurs.
DEF. _Metamorphosis_ obtected[1223].
_Mouth_ antliate[1224].
_Prothorax_ very short, covered by a pair of tippets.
_Wings_ four, covered partially or generally with minute
scales: _neuration_ branching, often with a central areolet.
10. DIPTERA[1225] (_Antliata_ F.). This Order likewise appears indebted
for its name to the philosopher of Stagyra, who distinguishes the
members of it from their counter-parts--the _Hymenoptera_--by their
having an _oral_, while these have an _anal_ sting[1226]: and we may
add, that while the last, on account of their wonderful economy and
the benefits which by them PROVIDENCE confers upon mankind, have been
justly regarded as the _princes_ of the winged insect world,--the
former, when we consider the filthy and disgusting habits of their
grubs, and the annoyance, both from their numbers and incessant
assaults, of them, in their fly-state, may very properly be considered
as its _canaille_. Almost all the tribes of _Hymenoptera_, from the
saw-flies to the ants, have their representatives in this Order. Though
the number of wings is its prominent feature, yet there are two-winged
insects in other Orders, as some _Ephemeræ_: and the _Eproboscidea_ of
Latreille seem rather a kind of winged _Aptera_, if we consider their
_trophi_, than real _Diptera_; or they may form an osculant group,
partly winged and partly apterous, between the two. I have before
remarked, that though, apparently, the insects of this Order have only
_two_ wings, yet the under or secondary wings of the other Orders have
in them their representative[1227]. Their poisers also, I formerly
observed to you, are probably more connected with their respiration
than with their flight[1228].
DEF. _Metamorphosis_ incomplete, or coarctate.
_Mouth_ proboscidate[1229].
_Prothorax_ replaced by a collar. _Sutures_ of the trunk
mostly spurious[1230].
_Wings_ two, with winglets attached to them: _neuration_
various[1231]. _Poisers._
_Tarsi_ pentamerous.
_Ovipositor_ various[1232].
11. APHANIPTERA[1233] (_Aptera_ L. Lamarck. _Rhyngota_ F. _Suctoria_
Latr.) This is an osculant Order, and is distinguished from the other
_Aptera_ L. in undergoing a regular metamorphosis. The larva is
vermiform, the pupa incomplete, and inclosed in a cocoon. Probably
the common flea and the chigoe would form distinct genera. The number
of species of fleas is greater than has been supposed. I have been
informed that Dr. Leach is acquainted with fourteen British species
alone. Besides their metamorphosis, they are distinguished from
the _Aptera_ by the number of segments into which their body is
divided, and by their pentamerous tarsi. Something like elytra and a
scutellum appear to distinguish these insects.
DEF. _Metamorphosis_ incomplete.
_Body_ apterous, compressed.
_Mouth_ rostrulate[1234].
_Tarsi_ pentamerous.
We are now come to those insects which, though they change their skin
in their progress to their state of perfection, and some of them, as
we have seen[1235], gain additional segments and pairs of legs, yet
none of them acquire wings or wing-cases: these I have considered as
forming one Order, under the denomination of
12. APTERA[1236] (_Synistata_, _Antliata_, _Unogata_, _Mitosata_ F.).
I do not give this as a _natural_ Order. Our knowledge, however,
of the internal organization of its groups, is not at present
sufficiently matured to warrant the formation of them into new
_Classes_[1237]: till that is more fully ascertained, it seems to me
therefore best to consider these groups as forming three _Suborders_:
the _first_ consisting of the _Hexapods_; the _second_ of the
_Octopods_; and the _third_ of the _Polypods_. It will be better, I
think, instead of giving a general character of the Order,--which
principally consists in the insects composing it being _Apterous_, or
never acquiring organs of flight,--to define each of these groups.
_Hexapods_ (_Ametabolia_ Leach, _Ametabola_ M^cL.). _Six_ legs may
be regarded as the natural number in _all_ the insect tribes[1238]:
but our business now is with those _Aptera_ whose body consists of
_three_ greater segments, and which in none of their states have ever
more or less than _six_ legs, and consist of the three Linnean genera
_Pediculus_, _Lepisma_, and _Podura_ (_Thysanura_ and _Anoplura_).
Some of the mites (_Acarus_ L.) are hexapods, but their body has no
distinction of head, trunk, and abdomen. The metamorphosis of most
female _Blattæ_, and of some other _Orthoptera_ that are apterous,
cannot be regarded as materially different from that of the Hexapods.
Amongst the _Anoplura_,--the _Pediculi_, or lice, are suctorious,
and the _Nirmi_, or bird-lice, masticators,--a circumstance which in
them does not appear to indicate even a different Order, and proves
that undue stress ought not to be laid, independently of general
characters, on the mode in which insects take their food.
DEF. _Metamorphosis_ complete.
_Body_ consisting of three principal segments.
_Mouth_ perfect, or rostellate[1239].
_Antennæ_ distinct.
_Legs_ six, in every state.
_Octopods._ This suborder consists of the _Trachean Arachnida_
of Latreille, excluding the _Pycnogonida_; of the _Acaridea_,
_Sironidea_, _Phalangidea_, and part of the _Scorpionidea_ of
Mr. MacLeay, and, with some exceptions, of the Linnean genera
_Acarus_ and _Phalangium_. This last tribe (for with Linné, I
include _Chelifer_ and _Obsidium_ in the _Phalangidea_,) on one
side approaches _Scorpio_ by _Thelyphonus_, and on the other the
_Aranidea_ by _Gonyleptes_; or, according to Mr. MacLeay, the
transit is to both by _Galeodes_[1240]. But as there is reason for
thinking that this last belongs to the _Pulmonary Arachnida_[1241],
and forms a peculiar type in that Class, I consider the transit
from the one to the other as above stated. The folded abdomen of
_Gonyleptes_ seems much to correspond with that of the _cancriform_
spiders (_Carkinodes cancriformis_, &c.).
DEF. _Metamorphosis_ complete.
_Body_ consisting of one or two principal segments.
_Mouth_ various[1242].
_Antennæ_ obsolete, or represented by mandibles.
_Legs_ mostly eight, but in a few six only[1243].
_Polypods._ This suborder consists of Dr. Leach's Class _Myriapoda_,
or the _Chilognatha_ and _Chilopoda_ of Latreille, corresponding
with the Linnean genera _Iulus_ and _Scolopendra_. Mr. MacLeay has
arranged them in the same Class with the Hexapods, and connects
them with the _Anoplura_ by means of certain intestinal worms of an
indistinct annulose structure[1244] (_Entozoa Nematoidea_ Rud.), in
which the sexes are diœcious, and some of which are furnished with
lateral spinulæ,--thus, as he supposes, connected with the Polypods;
and with the _Anoplura_ by others (_Epizoaria_ Lam.) in which
appendages appear somewhat analogous to the legs of Hexapods, as in
_Cecrops_ Leach, and which like them are parasitic animals[1245].
But the right of these worms to be considered as members of the
same Class with the Hexapods and Polypods at present appears rather
problematical, and requires further examination.
DEF. _Metamorphosis_ subcomplete[1246].
_Body_ consisting of numerous segments.
_Mouth_ perfect[1247].
_Eyes_ compound or aggregate.
_Antennæ_ distinct.
_Legs_ six on the trunk, many on the abdomen.
I must next say something on the Orders of the _Arachnida_. Every one,
at first sight, sees that _spiders_ and _scorpions_ are separated by
characters so strongly marked, that they look rather like animals
belonging to different Classes than to the same: these form the two
_primary_ Orders of the _Arachnida_, and they appear to be connected
by two _secondary_ or osculant ones,--on the one side by _Galeodes_,
and on the other by _Thelyphonus_ and _Phrynus_[1248]. This Class,
although there is an appearance of eight legs, is, strictly speaking,
of a _Hexapod_ type; for the anterior pair, ordinarily regarded as
legs and performing their function, are really the analogues of the
maxillary palpi of perfect insects. This will be evident to you if
you examine any species of _Galeodes_. These animals, if we look at
them cursorily, we should regard as _Decapods_; but when we trace
the two anterior pairs of apparent legs to their insertion, we find
that both proceed from the _head_, which in that genus is distinct
from the trunk; while the three last pairs, which alone are furnished
with claws, are planted, as legs usually are, in the latter part. The
first pair represent the ordinary palpi of _Arachnida_, are analogous
to the labial ones of Hexapods, and, as likewise in _Phrynus_ and
_Thelyphonus_, are more robust than what are usually taken for the
first pair of legs; but they differ in being considerably longer, and
instead of terminating in a _chela_ are furnished with a retractile
sucker[1249]. The second pair are more slender and shorter than the
first; they correspond precisely with what are deemed the first pair of
legs of _Octopods_ and _Arachnida_, and are clearly analogous to the
maxillary palpi of perfect insects. Whether the base of the first pair
of these palpi is in any respect analogous to the labium of insects,
(as that of the second seems to be to their maxillæ,) I am not prepared
to assert: it will therefore be most advisable to name these palpi
_anterior_ and _posterior_: but as they evidently proceed from the
_head_ in _Galeodes_, and in that genus are clearly analogous to those
of the _Phrynidea_, (which in their turn as clearly represent those of
the _Aranidea_,) it follows that in all they are organs of the part
representing the _head_, and therefore not in a _primary_ sense _legs_;
although in a _secondary_, as M. Savigny has proved, they may be so
called[1250].
1. ARANEIDEA M^cL. (_Aranea_ L., _Araneidæ_ Latr.) The _Araneidea_,
or spiders, seem resolvable into _two_ suborders,--the _Sedentaries_
and the _Wanderers_; thus forming, perhaps, what Mr. MacLeay would
denominate the normal groups of a circle of _Arachnida_.
DEF. _Mandibles_ armed with a perforated claw.
_Head_ and _Trunk_ coalite.
_Palpi_ pediform, anterior pair without claws.
_Abdomen_ without segments or elongated tail.
_Spiracles_ two[1251].
_Anus_ furnished with an apparatus for spinning[1252].
2. SCORPIONIDEA M^cL. (_Scorpio_ L. Latr.)
DEF. _Mandibles_ chelate.
_Head_ and _Trunk_ coalite.
_Anterior Palpi_ chelate[1253].
_Posterior Palpi_ pediform.
_Pectens_ two[1254].
_Abdomen_ divided into segments and terminating in a jointed
tail, armed at the end with a sting[1255].
_Spiracles_ four pairs.
3. GALEODEA.
DEF. _Head_ distinct[1256].
_Eyes_ two.
_Mandibles_ chelate with dentated chelæ.
_Palpi_ pediform, the anterior pair thickest with a
retractile sucker.
_Trunk_ consisting of two principal segments, with a minute
supplementary posterior one[1257].
_Spiracles_ two placed in the trunk[1258].
_Pseudo-pectens_ two[1259].
_Abdomen_ divided into segments.
_Anus_ unarmed and without a spinning apparatus[1260].
4. PHRYNIDEA.
DEF. _Mandibles_ unguiculate.
_Anterior Palpi_ chelate or unguiculate[1261], very robust.
_Posterior Palpi_ pediform, very long and slender.
_Abdomen_ divided into segments.
_Spiracles_ two pairs.
_Anus_ terminating in a mucro, and sometimes in a filiform
jointed tail without a sting at the end.
v. Having considered the _Orders_ into which _Insecta_ and
_Arachnida_ may be divided, I am next to give you some account of the
_groups_ into which each is further resolvable. To draw out, however,
a complete scheme of these would be deviating from my province,
and extend this letter to an enormous length. Indeed, to give the
_natural_ primary and subordinate sections of every Order, would
require a knowledge of the subject to which no Entomologist has yet
attained. I shall therefore only say something general upon them, and
refer you to an example of each kind of group.