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and postscutellum are often continued laterally on either side
imtil they join the pleuron, although frequently these lateral
prolongations become much reduced in size or split up into
smaller pieces (Fig. 2, prsc, psl). Projecting internally, some-
times from the anterior margin of the tergum, sometimes from
the posterior margin, or from both, is a process termed the
phragma, which serves for the attachment of muscles. When
the phragma is attached to the anterior margin of the tergum, it
is termed the prephragma and when attached to the posterior
margin, is termed the postphragma (Snodgrass, '10). The
prescutum and postscutellum are frequently represented in the



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50 Annals Entomological Society of America [Vol. IX,

tergum of insects simply by the inward projecting phragmas.
In some insects the postscutellum is completely separated from
the scutoscutellum by membrane. (Fig. 10, psl).

The old view that the tergum of adult insects consists of four
consecutive transverse plates was proposed by Audouin, '24.
He termed these plates the presecutum, scutum, scutellum and
postscutellum, and this terminology has been retained though
Audouin's general conception of the tergum, held until recently
by most entomologists, is now obsolete. Other writers have
given various names to the different tergal regions. Amans, '85,
proposed the terms prodorsum, dorsum, postdorsum and sub-
postdorsum for the identical sclerites previously termed pre-
scutum, scutum, scutellum and postscutellum by Audouin.
Audouin's terminology has the right of priority, is widely
accepted, has no undesirable features and should therefore stand.

Wings. The wings of insects arise in two ways. In hemi-
metabolous insects they appear as outgrowths at the lateral
margins of the meso- and metanotum. In holometabolous
forms, they arise internally as the so-called wing-buds, appear-
ing externally at or after pupation. At first the wings are filled
with tracheae, blood, tissue, etc. The tracheae persist and are
ultimately replaced by the veins or nervures of the adult insect
wing, which serve as stiffening supports for the wing mem-
brane. The blood and tissue gradually disappear and the two
lateral surfaces of the sac-like outgrowth come together, forming
the wing membrane.

The wings of adult insects are connected with the scuto-
scutellum along its lateral margin, on the lateral margin of the
tergum. They articulate with the pre- and post-alar processes,
together with the small alar sclerites at the base of the wing
(Fig. 10, pra, poa, sa, ba). Beneath they articulate with
the pleural wing process (Fig. 10, plwp).

The wings are variously modified in different insects,
forming the elytra of Coleoptera, the tegmina of Orthoptera,
the halteres of Diptera, the hemielytra of Hemiptera and the
hairy and scaly wings of Lepidoptera and Trichoptera. In
such forms as Carabidae, Ptinidae, and weevils the hind wings
are often lacking, owing to disuse, and the elytra in some forms
fuse together thereby forming a solid covering for the hind
wings (if present) and the abdomen. The general belief
has always been that elytra are modified wings.



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1916] Thoracic and Cervical Scleriies of Insects 51

In homologizing the wings in the different orders of insects,
by their venation, Comstock made use of the tracheae, which
occupy the position afterwards assumed by the principal veins
in the wings of generalized insects. He termed the veins
costa, subcosta, radius, media, cubitus, and anals. This
terminology is quite widely accepted and generally used,
especially for Lepidoptera.

Several theories have been proposed to account for the
origin of the wings. Gegenbauer, 70, claimed that the wings
developed from tracheal gills. Palmen, '77, discredited this
theory by demonstrating that tracheal gills occur on the
sternum, abdomen, tergum, pleuron and in the anal region;
that tracheal gills and the closed tracheal system is a secondary
adaptation to the aquatic life of the larva and that aerial res-
piration was probably the primitive condition.

Plateau, *71, thought that the wings developed from hyper-
trophied spiracles. Muller, '75, from a study of the develop-
ment of the wings of Calotertnes, concluded that they arose as
lateral outgrowths of the dorsum. To this theory Pancritius,
'84, adds the idea, that the primitive outgrowth of the body wall
may have developed into a protective body covering like an
elytron, which became modified to form the wings. Packard,
'98, accepted and developed Muller's theory. He apparently
thought that primitive winged insects had lateral extensions
of the thoracic segments, which acted as a sort of parachute
and which later gave rise to true wings. Palaeontological
records show that some insects had lateral extensions of the
pronotum which may have served as a parachute and that many
of the earliest of the Pterygota have well developed wings,
which seem to have articulated with the thorax. Packard's
theory is plausible.

Pleuron. The pleuron in a restricted sense consists of
the sclerites lying between the dorsum and sternum and forming
the lateral wall of any thoracic segment. In nearly all insects
it is composed of two sclerites, the episternum or anterior
sclerite and the epimeron or posterior sclerite (Fig. 8, esi, emi).
In the higher forms, the pleuron is usually connected with the
tergum by prolongations of the prescutum and postscutellum,
the latter often extending downward for some distance and
fusing with the pleuron, in which case it is frequently mistaken



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52 Annals EntomologiccU Society of America [Vol. IX,

for a part of the pleuron (Figs. 2 and 11, psl, prsc). These
prolongations of the tergum are usually lacking in the lower
forms. In other insects they may be much reduced in size
or split up into smaller pieces.

The epistemum and epimeron are separated externally by a
suture, termed the pleural suture, which extends from the
pleural wing process or fulcrum to the pleural coxal process
(Pig. 1, plsa). Internally these two sclerites may be readily
distinguished from each other by a strongly chitinized ridge
variously termed the pleural ridge, entopleuron or apodeme,
which likewise extends from the pleural wing process to the
pleural coxal process.

The pleuron bears three processes which usually have an
inward projection for the attachment of muscles. Above is the
pleural wing process which serves for the articulation of the
wing (Fig. 2, plwp). It consists of a dorsal prolongation of the
pleuron, of variable length, through which runs the pleural
suture externally and the entopleuron internally. On the lower
margin of the pleuron is the pleural coxal process, with which
the coxa articulates (Fig. 4, cxp). It is similar to the pleural
wing process and has the pleural suture and pleural ridge
extending through it. The pleural process or pleural arm
(Snodgrass, *10) is situated a short distance above the coxal
process. It projects inward and downward from the ento-
pleuron. Usually it rests against the furca (an inward pro-
jecting process of the sternum, termed the apophysis) and
frequently fuses with it (Fig. 12, f).

Along the dorsal edge of the pleuron are generally foimd
three or four small sclerites. Two of these, the basalar plates
(Crampton, *14) are generally found in front of the pleural
wing process, and are termed by Crampton, '14, the anterior
and posterior basalar sclerites, terms which will be used in this
paper (Fig. 2, pba, aba). Behind the wing fulcrum there is
usually one, sometimes two of these sclerites, which are termed
subalar plates (Crampton, '14, Fig. 2, sa). Lowne, '90, in his
book on the Blow-Fly, terms the subalar plate, the **costa. "
Snodgrass, *08, terms it the **postepimeron" and Crampton, '09,
the posterior '* costal sclerite. " Verhoeff termed the anterior
basalar plate, the **alarpleura. " Snodgrass, '10, termed all
these plates the *'paraptera. " Those situated in front of the
wing process, he called the preparaterum or epistemal paraptera



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1916] Thoracic and^ Cervical Sclerites of Insects 53

and those behind the postparapterum or epimeral paraptera*
Snodgrass claims in his "Anatomy of the Honey Bee," pp. 20,
footnote (a), that Audouin, '24, termed these plates the parap-
tera and hence Snodgrass would retain this term. This,
however, is not the case. Audouin, '24, termed the small
sclerite which sometimes extends along the anterior edge of the
epistemum the **paraptere" arid confuses it with the tegulae
and other sclerites at the base of the wing. Later Audouin, '32,
clearly defined the ''paraptere" as the tegulae (1832 Audouin,
Exposition de L'anatomie du Thorax, par W. S. MacLeay,
Accompagnee de notes par M. Audouin. Ann. Sci. Nat.
XXV, Ser. 1, pp. 41, footnote.) He says: '*In fact I consider
as the paraptere the small piece so visible in the Hymenoptera
and in the Lepidoptera, which covers the base of the fore wings
and which has been designated by the name ecaille, epaulette or
squamala. " Jordan, '02, terms the subalar plate the ** para-
sternum" and also applies this term to the anepistemum
(upper portion of the epistemum) in his figures. His homologies
of the sclerites are evidently incorrect. Berlese, *06, termed the
basalar sclerites the *'acrostemo o prefulcro (anteriore e
posteriore)." The subalar sclerite he terms the **paraptero,"
(GU Insetti, pp. 244). ''Prefulcro" is a good term for the
basalar sclerites, as it designates their position in front of the
wing fulcrum, but the term *'paraptero" does not seem desir-
able. Crampton's, '14, term, subalar sclerites, is far more
suitable since it exactly describes the position of these sclerites.

The pleuron may be more or less modified in different
insects. In the mesothorax of some Diptera, the parts have
shifted forward, thereby causing the pleural suture to become
twisted and curved (Fig. 11, pls2). In Odonata the pleuron
assiunes an oblique position and as a result, the epistemum
becomes dorsal and the epimeron ventral. In other insects the
pleural sclerites are variously modified in shape and size.

Crampton, '08, was the first to suggest that the pleuron
probably consisted originally of but one plate and that the
epistemum and epimeron may have been formed by the infolding
of the integument, due to muscle stress. This view was later
developed by Snodgrass, '09. In such insects as Eosentomon,
Leuctra (prothorax), acridid nymphs and Anisolabis, the pleuron
is apparently represented by a single plate, in which the pleural
suture is present. If the pleuron was originally a single plate,



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54 Annals Entomological Society of America [Vol. IX,

which seems very probable, then the pleural suture probably
arose by an infolding of the integument of this plate. In the
pleuron of Periplaneta for example, the line which represents
the pleural suture is clearly formed by the two external lips of
the infolded integument coming together and resting against
each other, but the edges or lips do not fuse. On pressing the
edges or lips of the fold apart, a large hollow pocket will be seen,
at the bottom of which is the apodeme and the pleural arm.
The pleural arm is sometimes hollow and usually chitinized.
These conditions would be produced by the infolding of the
integument of the pleuron, possibly due to muscular stress,
possibly from other causes. This is the simplest and most
probable explanation.

Some investigators regard the pleuron as being formed by
two sclerites coming together, fusing and the edges rolling inward
to form the pleural suture and apodeme. This view seems very
improbable. It is complicated and does not account for the
single plate representing the pleuron in Eosentomon, Leuctra,
etc. Furthermore, two sclerites whose edges meet and fuse are
not apt to be plastic enough to permit their being drawn out
into a prolongation forming the pleural arm and pocket, but
would more probably be firm and resistant.

Audouin, '24, considered the pleuron as composed of three
sclerites, the anterior or epistemum, the posterior or epimeron
and the peritreme or spiracle bearing sclerite. This terminology
is widely accepted (with the exception of peritreme) in general
use and there is no valid reason for changing it as some writers
have done, since this only leads to confusion. Kirby refers to
the epimeron as the pleura. Burmeister thought the pleuron
was part of the sternum and others regard it as the basal
sclerites of the leg.

Heymons, *99, in his **Beitrage zur Morphologie und Ent-
wichlungsgeschichte der Rhynchoten,'* pp. 443, in discussing
the sclerites of Nepa, was the first to use the term subcoxa and
appHed it to what he thought was the mesothoracic pleuron,
consisting of epistemum and epimeron. A study of Nepa shows
that Heymons* subcoxa actually consists of the mesothoracic
epistemum, the precoxal bridge or precoxale (Crampton, '14),
and possibly the trochantin. The sclerite in Nepa which
Heymons thinks is the metathoracic pleuron, he terms the
**pleurite,'' pp. 376. This sclerite is not the metathoracic



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1916] Thoracic and Cervical Sclerites of Insects 55

pleuron, but is the epimeron of the mesothorax. It extends
posteriorly as a sort of flap covering the metathoracic epimeron,
which would easily escape notice unless closely observed.
Heymons was confused in the use of his own term subcoxa,
which has led to its misinterpretation by other workers. Ender-
lein and also Berlese thought the ** subcoxa" corresponded to
the trochantin. Bomer considered it equivalent to his *'mero-
stemum'* or pleuron. Verhoeff regarded it as representing the
coxopleure (epistemum) and trochantin.

Epimeron. The epimeron varies greatly in size and shape
in different insects. Most writers consider it as a single sclerite,
which is usually the case (Fig. 8, emi).

In the prothorax of Periplaneta there is a small sclerite,
which has become split off from the posterior edge of the
epimeron, and may be termed the postepimeron (Fig. 3, pemi).
Snodgrass, '08, applied this term to the subalar plate. It has
since been discarded and so far as I know, has never been
applied to any other sclerite. Therefore, as it describes the
exact position of the sclerite, which has become detached from
the posterior edge of the epimeron, I have adopted it to designate
the sclerite in question. In the prothorax of Capnia (Fig. 4,
pemi), there is a large sclerite comparable to the postepimeron,
which is likewise split off from the posterior edge of the epimeron
and extends behind the coxa until it meets the furcasteriiite,
thereby forming the postcoxal bridge or postcoxale (Crampton,
'14). In the mesothorax of Corydalis (Fig. 10, pemj) the
epimeron is a single sclerite, a projection of which extends
behind the coxa. In many Coleoptera and Tenthredinidae
(prothorax of Dolerus (Fig. 12, emi) the epimeron is greatly
reduced in size.

The epimeron is often divided into two sclerites by a trans-
verse suture. This condition can be seen in the mesothorax of
such insects as Mantispa, Chrysopay Leptis, and in some
Tipulidae and Tabanidae (Figs. 1 and 2, emj). In Diptera the
lower portion of the epimeron of the mesothorax is usually fused
with the meron, being separated by a suture, although the
suture is often wanting (Figs. 1 and 11, mp). Packard, '80,
seems to have been the first to apply terms to the subdivisions
of the epimeron. In describing the thorax of Mantispa, he
termed the upper region, the sur-epimeron and the lower the



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56 Annals Entomological Society of America [Vol. IX,

infra-epimeron. Crampton, '08, used the terms hyper- and
hypo-epimeron for these regions, but on account of the sim-
ilarity of the terms he later (Crampton, '09), discarded them,
substituting in their place anepimeron and katepimeron.

Osten-Sacken's terminology has been widely used for the
Dipteron thorax and is very good. Crampton, '14, retains it in
a slightly modified form, viz. : changing the ending from pleura
to pleurite. Thus in the Diptera, Crampton, '14, terms the
upper region of the epimeron the pteropleurite (situated beneath
the wing base (Fig. 1, ptp) and the lower region meropleurite
(usually fused with the meron, especially in Diptera (Pigs. 1
and 11, mp). These terms will be adopted in this paper.

Some Diptera such as Tipula, Chrysopa, etc., have the lower
portion of the epimeron separated from the meron by a suture.
In such cases the term katepimeron (Crampton, *09) will be
applied to the lower portion of the epimeron (Figs. 1, 2 and 13,
kem). The term meropleurite will be applied to the katepimeron
plus the meron whether these sclerites are completely fused into
one as in the mesothorax of Tabanus (Fig. 11, mp), or separated,
by a suture as in Tiptda and Chrysopa (Figs. 1 and 13, mp). In
either case the meropleurite represents identical regions (Figs.
11 and 13, mp).

The subalar plate or its representative is always present
behind the pleural wing process (Figs. 1, 2 and 11, sa). It may
be partly fused with the epimeron or entirely separated from
it by membrane. Both of these conditions can be found in the
Diptera (Leptis, Tabanus, etc., Figs. 1 and 2, sa).* Sometimes
the subalar plate is divided into two sclerites as in some of the
Plecoptera, and in that case will be termed anterior and poster-
ior subalar sclerites (Crampton, '14). In some Leptidae and
Tabanidae, there is a cleft immediately below the subalar
plate (Figs. 2 and 11, sa), which is prolonged downward into
the epimeron (pteropleurite, ptp) for a short distance in the
form of a suture, but the internal ridge of this suture is not
continuous with the pleural suture. Snodgrass, '10, mistook
this suture for the pleural suture, which it closely resembles
when viewed externally. By carefully examining a specimen
which has been boiled in caustic potash, the mistake will be
readily discovered and the real pleural suture can be easily
traced by means of the apodeme, from the coxal process into
the pleural wing process (Figs. 2 and 11, pls2).



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X9161 Thoracic and Cervical Sclerites of Insects 57

In studying the thorax in a series of insects such as Mantispa,
Chrysopa, Tipula, Leptis and TabanuSy it will be observed that
there is a gradual shifting forward of the sclerites, being most
pronounced in the mesothorax (Figs. 13, 1, 2 and. 11). This
causes the pleural suture of the mesothorax to become more or
less crooked as shown in Figs. 1, 2 and 11, pls^.

Starting at the upper end of the mesothoracic wing fulcrum
of Tabanus (Pig. 11, plwp) the pleural suture (plss) nms down-
ward and slightly forward in a more or less curved line until
it meets the stemopleurite (lower region of the epistemum).
(Pig. 11, stp). Here it turns almost at right angles and nms
posteriorly to the meropleurite (lower portion of epimeron),
(Pig.. 11, mp) and thence downward and slightly backward to-
the coxa. This condition is very confusing and hard to see,
imless the specimen is first boiled in caustic potash, in which
case the pleural suture is readily observable.

In Diptera the meron is often fused with the lower region
pf the epimeron, forming the meropleurite (Pig. 11, mp), but
as the former sclerite is usually closely connected with the coxa
it will be treated under the heading Coxa.

Audouin, '24, applied the term epimeron to the sclerite
immediately posterior to the epistemum. This term has been
generally accepted and used by nearly all subsequent ento-
mologists. The epimeron was termed by Burmeister, '32, the
''pleura;" by Verhoeff, '03, the *'anopleure;" by Amans, '85,
the **postpleuron," and by Heymons, '99, the **pleurite. "
Hammond, Brauer and Lowne who have worked on the Dip-
teron thorax, where the epimeron is often subdivided into
two sclerites, have applied the term epimeron to various
thoracic sclerites and the terminology has been greatly confused.
Osten-Sacken's terminology for the thorax of Diptera is very
good and should be retained. Crampton, '14, in a review of
the Dipteron thorax has retained this terminology slightly
modified as previously stated. This terminology should also
be applied to all insects in which the epimeron and epistemum
are subdivided as in Mantispa, Chrysopa, etc., since it has been
established by wide-spread acceptance among those working
on Diptera and should be made a uniform terminology for all
insects with a thorax of this nature, as far as possible.

The term parapleuron has given rise to much confusion
among morphologists. It was first applied to the entire pleural



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58 Annals Entomological Society of America [Vol. IX^

region by Knoch. Kirby termed the epimeron and epistemum^
the parapleuron, an interpretation accepted by Smith, '06.
Burmeister and also Voss applied the term parapleuron to the
epistemum. Kolbe used it for a sclerite occurring in beetles,
situated behind the wing and between the tergum and pleuron.
Ritter's parapleuron is equivalent to the anepistemum or
upper region of the epistemum. Others have used the term
parapleuron to designate various sclerites of the thorax. At
present there is no uniformity of opinion concerning the applica-
tion of the term even among Coleopterologists, by whom it is
chiefly used.

Episternum. The epistemum is also subject to great
variation in insects. It may consist of a single sclerite as in
the earwig (Fig. 8, esi), or it may be subdivided into an upper
and lower region as in Mantispa, Chrysopa, Corydalis, Tipula,
etc. (Pig. 2, es2). The suture which divides the episternum
into two regions may extend clear across, making a complete
division, as in the mesothorax of Tabanus (Fig. 11, g) or only
part way, as in the mesothorax of Tipula (Fig. 1, g). In
Chrysopa and Mantispa (Fig. 13), the division is represented
by a narrow strip of chitin of varying width, extending inward
midway between the two subdivisions. This strip is probably
a part of the epistemum which has become fused with the
precoxal bridge (Crampton, '14). It may, however, belong
entirely to the precoxal bridge, a projection of which extends
between the two subregions, but this latter alternative is very
improbable. The formation of this region in other insects
seems to indicate that it is a part of the epistemum. Crampton,
'09, applied the term ** median region," to this narrow strip of
chitin. This seems to be the only term which has been applied
to it. It hardly seems necessary to give this region a name
and consequently none will be used in this paper.

When the episternum is subdivided, a condition found in a
great many insects, the lower division may be fused with the
sternum (Figs. 1, 2 and 11, stp). The epistemum is separated
from the epimeron by the pleural suture. In all insects, both
the epistemum and epimeron extend from the coxal process to
the pleural wing process. This condition can be plainly seen
in most insects, but in some such as Tabanus, etc. (Fig. 11, stp),
the lower portion of the epistemum of the mesothorax has



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1916] Thoracic and Cervical Sclerites of Insects 59

become fused with the sternum. Many workers have con-
sidered this fusion product of sternum and lower portion of the
epistemum as the entire sternum and accordingly termed it the
sternum. This misinterpretation has led to great confusion in
the terminology. In the mesothorax, Lowne termed this
composite region the **mesoplastron. *' He also applied the
term, **metaplastron*' to the meropleurite (lower portion
of epimeron fused with the meron) of the mesothorax (Fig.
11, mp). Chabrier had originally used the term '* plastron"
to designate the pleuron of the prothorax, so that Lowne is
incorrect in stating that the ''mesoplastron" is the '* plastron"
of Chabrier. Packard, '80, used the terms sub- and infra-
episternum to designate the fusion product of the sternum and
lower region of the epistemum. Snodgrass, '10, and several
other workers termed it the '* sternum. " Osten-Sacken, '89,
used the term **stemopleura. " Crampton, '09, designated it
by the component parts entering into its composition, but later
Crampton, '14, adopted Osten-Sacken's terminology, slightly
modified, calling it the sternopleurite. The latter term will
be used in this paper.

The lower portion of the epistemum in Chrysopa and like
insects has a new region marked off, which is composed of a
different combination of sclerites than the region representing



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