Guide to the study of insects, and a treatise on those injurious and beneficial to crops: for the use of colleges, farm-schools, and agriculturists online

crowded down out of sight, and placed between the two oppos-
ing rings. As seen in Fig. 11, the praescutum of the moth is
a small rounded piece, bent vertically down, so as not to be
seen from above. In the lowly organized Hepialus, and some

FIG. 11. Tergal view of the middle segment of the thorax of Telea Polyphemus,
prm, praescutum; ms, scutum; scm, scutellum; ptm, postscutellum; pt, patagium,
or shoulder tippet, covering the insertion of the wings. Original.

FIG. 12. Side view of the thorax of T. Pohjphemus, the hairs removed. 1, Pro-
thorax ; 2, Mesothorax ; 3, Metathorax, separated by the wider black lines. Tergum
of the prothorax not represented, ms, mesoscutum; scm, mesoscutellum; ms" ,
metascutum; scm'", metascutellum; pt, a supplementary piece near the inser-
tion of patagia; w, pieces situated at the insertion of the wings and surrounded by
membrane ; em, epimerum of prothorax, the long upright piece above being the
episternum; epm", episternum of the mesothorax; em", epimerum of the same;
epm", episternum of the metathorax; em", epimerum of the same, divided into two
pieces; c, c", c'", coxa?; te, le", le", trochantines ; tr, tr, tr, trochanters.
Original.





12 THE CLASS OF INSECTS.

Neuroptera, such as the Polystaxhotes (Fig. 13 a), the prae-
scutum is large, well developed, triangular, and wedged in
between the two halves of the scutum. The little ,
piece succeeding the scutellum, i. e. the postscu-
tellum, is still smaller, and rarely used in descrip-
tive entomology. Thus far we have spoken of the 3 |
middle, or mesothoracic, ring, where these four
pieces are most equally developed. In the" first,
or prothoracic, ring, one part, most probably the
scutum, is well developed, while the others are
aborted, and it is next to impossible to trace them
in most insects. The prothorax in the higher in- Fi s- 13 -
sects, such as the Efymenoptera, Lepidoptera, and Diptera is
very small, and often intimately soldered to the succeeding or
mesothoracic ring. In the lower insects, however, such as the
Coleoptera, the bugs (Hemiptera), grasshoppers and their
allies (Orthoptera), and the Neuroptera, the large broad pro-
thorax consists almost entirely of this single piece, and most
writers speak of this part under the name of "thorax," since
the two posterior segments are concealed by the wings when
the animal is at rest. The metathorax is usually very broad
and short. Here we see the scutum split asunder, with the
prsescutum and scutellum wedged in between, while the post-
scutellum is aborted.

On the side are two pieces, the upper (epimerum) placed
just beneath the tergum, which is the collective name for the
four tergal, or dorsal, pieces enumerated above. In front of
the epimerum and resting upon the sternum, as its name im-
plies, is the episternum. These two parts (pleurites) compose
the flanks of the elemental ring. To them the legs are articu-
lated. Between the two episterna is situated the breast-piece
(sternum), which shows a tendency to grow smaller as we
ascend from the Neuroptera to the Bees.

In those insects provided with wings, the epimera are also
subdivided. The smaller pieces, hinging upon each other, as
it were, give play to the very numerous muscles of flight

FIG. 13. A tergal view of thorax of Hepialus (Sthenopis) ; 1, prothorax ; 2, meso-
thorax; 3, metathorax. The prothorax is very small compared with that of Poly-
stcechotes (13 a, 1), where it is nearly as long as broad. Original.



COMPOSITION OF THE INSECT-CRUST. 13

needed by the insect to perform its complicated motions
while on the wing.

The insertion of the fore wing is concealed by the ''shoulder
tippets," or patagia (Fig. 11), which are only present in the
mesothorax. The external opening of the spiracles just under
the wing perforates a little piece called by Audouin the peri-
treme.

A glance at Figures 11 and 12 shows how compactly the
various parts of the thorax are agglutinated into a globular
mass, and that this is due to the diminished size of the first
and third rings, while the middle ring is greatly enlarged to
support the muscles of flight. There are four tergal, four
pleural, two on each side (and these in the Hymenoptera, Lepi-
doptera, and Diptera subdivide into several pieces), and a
single sternal piece, making nine for each ring and twenty-
seven for the w r hole thorax, with eight accessory pieces (the
three pairs of peritremes and the two patagici) , making a total
of thirty -five for the entire thorax ; or, multiplying the four
tergal pieces by two, since they are formed by the union of two
primitive pieces on the median line of the body, we have
thirty-nine pieces composing the thorax.

TABLE OF THE PARTS OF THE THORAX APPLIED TO THE PRO-,
MESO-, AND METATHORAX, RESPECTIVELY.

, Praescutum,
Dorsal S Scutum,
Surface j Scutellum,

* Postscutellum.



Episternal apophysis, Stigma, Peritreme.



We must remember that these pieces are rarely of precisely
the same form in any two species, and that they differ, often in
a very marked way, in different genera of insects. How sim-
ple, then, is the typical ring, and how complex are the va-
rious subdivisions of that ring as seen in the actual, living
insect, where each part has its appropriate muscles, nerves, and
tracheae !

We have seen how the thorax is formed in Insects generall}',
let us now advert to the two types of thorax in the six-footed



14 THE CLASS OF INSECTS.

insects. In the higher series of suborders, comprising the Dip-
tera, Lepidoptera and Hymenoptera, placing the highest last,
the thorax shows a tendency to assume a globular shape ; the
upper side, or tergum, is much arched, the pleural region bulges
out full and round, while the legs conceal at their insertion
the sternum which is minute in size.

In the lower series, embracing the Coleoptera, Hemiptera,
Orthoptera, and Neuroptera, the entire body tends to be more
flattened ; in the thorax the tergum is broad, especially that of
the prothorax, while the pleurites (episterna and epimera) are
short and bulge out less than in the higher series, and the ster-
num is almost invariably well developed, often presenting a
large thick breast-plate bearing a stout spine or thick tubercle,
as in (Edipoda. We can use these characters, in classifying
insects into suborders, as they are common to the whole order.
Hence the use of characters drawn from the wings and mouth-
parts (which are sometimes wanting), leads to artificial dis-
tinctions, as they are peripheral organs, though often convenient
in our first attempts at classifying and limiting natural groups.

The abdomen. In the hind body, or third region of the
trunk, the three divisions of the typical ring (arthromere) , are
entire, the tergum is broad and often not much greater in ex-
tent than the sternum ; and the pleurites also form either a
single piece, or, divided into an epimerum and episternum,
form a distinct lateral region, on which the stigmata are sit-
uated. The segments of the abdomen have received from
Lacaze-Duthiers a still more special name, that of urite, and
the different tergal pieces belonging to the several rings,
but especially those that have been modified to form the genital
armor have been designated by him as tergites. We have
applied this last term to the tergal pieces generally. The typi-
cal number of abdominal segments is eleven. In the lowest
insects, the Neuroptera, there are usually eleven ; as we have
counted them in the abdomen of the embryo of Diplax. In
others, such as the Hymenoptera and Lepidoptera, there may
never be more than ten, so far as present observation teaches
us.

The formation of the sting, and of the male intromittent
organ, may be observed in the full-grown larva and in the in-



COMPOSITION OF THE OVIPOSITOR.



15






Fig. 16.



complete pupa of the Humble-bee, and other thin-skinned

Ilymcnoptcrous larvae, and in a less satisfactory way in the

yon 1112; Dragon-flies.

If the larva of the Humble-bee be taken just after it has

become full-fed, and as it is about to enter upon the pupa state,

the elements
(sterno - rhab-
dites Lacaze-
Duthiers), or
tubercles,
destined to

form the ovipositor, lie in
separate pairs, in two groups,
Fig. 14. Fig. is. exposed distinctly to view,

as in Figures 14-18. The ovipositor thus consists of three

pairs of slender non-articulated tubercles, situated in juxta-
position on each side of

the mesial line of the

body. The first pair arises

from the eighth abdominal

ring, and the second and

third pair grow out from

the ninth ring. The ends

of the first pair scarcely

reach beyond the base of

the third pair. With the

growth of the semi-pupa,

the end of the abdomen

decreases in size, and is



Fig. 17.



17 a.




Fig. 18.



IS a.



FIG. 14. Rudiments of the sting, or ovipositor, of the Humble-bee. 8, 9, 10,
Bternites of eighth, ninth, and tenth abdominal rings in the larva, a, first pair, situ-
ated on the eighth sternite ; 6, second and inner pair ; and c, the outer pair. The let-
tering is the same in figures 14-22. The inner pair <6), forms the true ovipositor,
through which the eggs are supposed to pass when laid by the insect, the two
outer pairs, a and c, sheathing the inner pair.

FIG. 15. The same a little farther advanced.

FIG. 1G. The same at a later stage, the three pairs approximating.

FIG. 17. The three pairs now appear as if together growing from the base of the
ninth segment: 11 a, side view of the same, showing the end of the abdomen grow-
ing smaller through the diminution in size of the under side of the body.

FIG. 18. The three pairs of rhabdites now nearly equal in size, and nearly
ready to unite and form a tube; 18, sifle view of the same; the end of the abdo-
men still more pointed; the ovipositor is situated between the seventh and tenth
ring?, and L-, partially retracted within the body.



16



THE CLASS OP INSECTS.





gradually incurved toward the base (Fig. 18), and the three
pairs of rhabdites approach each other so closely that the two
outer ones completely ensheath the inner, until a complete
extensible tube is formed, which is gradually withdrawn entirely
within the body.

The male genital organ is originally composed of three pairs

(two pairs, apparently, in ^Es-
jL c/ma, Fig. 19) of tubercles all
arising from the ninth abdominal
ring, being sternal outgrowths
and placed on each side of the
mesial line of the body, two be- Fi - 2 -
ing anterior, and very unequal in size, and the
rig. 19. third pair nearer the base of the abdomen. The ex-
ternal genital organs cannot be considered as
in any way homologous with the limbs, which
are articulated outgrowths budding out be-
tween the sternal and pleural
pieces of the arthromere.*
~ b This view will apply to the
Fig. 21. genital armor of all Insects, so
far as we have been able to observe. It is
so in the pupa of ^Esclma (Fig. 21), and
the pupa of Agrion (Fig. 22), which com-
pletely repeats, in its essential features, the
structure of the ovipositor of Bombus. Thus in ^Eschna and
Agrion the ovipositor consists of a pair of closely appressed ensi-
form processes which grow out from under the posterior edge of
the eighth abdominal ring, and are embraced between two pairs

* This term is proposed as better defining the ideal ring, or primary zoological
element of an articulated animal than the terms somite or zoonite, which seem too
vague ; we also propose the term arthroderm for the outer crust, or body walls, of
Articulates, and arthropleura for the pleural, or limb-bearing region, of the body,
being that portion of the arthromere situated between the tergite and sternite.

FIG. 19. The rudiments of the male intromittent organ of the pupa of ./Eschna,
consisting of two flattened tubercles situated on the ninth ring; the outer pair
large and rounded inclosing the smaller linear oval pair.

FIG. 20. The same in the Humble-bee, but consisting of three pairs of tubercles,
x, y, z ; 8, 9, 10, the last three segments of the abdomen.

FIG. 21. The rudimentary ovipositor of the pupa of ^Eschna, a Dragon-fly.

FlG. 22. The same in pupa of Agrion, a small Dragon-fly. Here the rudiments
of the eleventh abdominal ring is seen, d, the base of one of the abdominal false
gills. Figs. 14-22 original.





Fig. 22.



COMPOSITION OF THE OVIPOSITOR.



17




of thin lamelliform pieces of similar form and structure, arising

from the stcrnite of the ninth ring. These sternal outgrowths

do not homologize with the filiform, antennae-like, jointed

appendages of the eleventh ring, as seen in the Perlidae and

most Neuroptera and Orthoptera (especially in Mantis tes-

sellata where they (Fig. 23) closely

resemble antennae), which, arising as

they do from the arthroplcural, or limb-

bearing region of the body, i. e. between Fig. 23.

the sternum and episternum, are strictly homologous with the

abdominal legs of the Myriapoda, the "false legs" of cater-

pillars, and the abdominal legs of some Neuropterous larvae

(Corydalis, Phyganeidce, etc.).

It will thus be seen that the attenuated form of the tip is
produced by the decrease in size of certain parts, the actual
disappearance of others, and the perfection of those parts to
be of future use. Thus towards the extremity of the body
the plcurites are absorbed and disappear, the tergites overlap
on the sternites, and the latter diminish in size and are
withdrawn within the body, while the last, or eleventh sternite,
entirely disappears.* Meanwhile the sting grows larger and

larger, until finally we
have the neatly fashioned
abdominal tip of the bee
concealing the complex
sting with its intricate
system of visceral ves-
sels and glands.
The ovipositor, or sting, of all insects, therefore, is formed
on a common plan (Fig. 24). The solid elements of the arthro-

*In limuttra, however, Lacaze-Duthiers has noticed the curious fact that in
order to form the long respiratory tube of this insect, the tergite and sternite of the
pregenital (eighth) segment are aborted, M'hile the pleurites are enormously en-
larged and elongated, so as to carry the stigmata far out to the end of the long tube
thus formed.

FIG. 23. End of the abdomen of Mantis tessellata; p, many-jointed anal style
resembling an antenna. 5-11, the seven last abdominal segments; the 8-llth ster-
nites being obsolete. From I.acd-.i-Diitliiers.

FIG. 24. Ideal plan of the structure of the ovipositor in the adult insect. 1-7*,
t IK tergites, connected by dotted lines with their corresponding sternites. 6, the
eighth tergite, or anal scale; c, epimerum ; a, a, t\vo pieces forming the outer pair
of rhabdites; i, the second pair, or stylets; and /, the inner pair, or sting; d, the

2




. 24.



18 THE CLASS OF INSECTS.

mere are modified to form the parts supporting the sting alone.
The external opening of the oviduct is always situated between
the eighth and ninth segments, while the anal opening lies at
the end of the eleventh ring. So that there are really, as
Lacaze-Duthiers observes, three segments interposed between
the genital and anal openings.

The various modifications of the ovipositor and male organ
will be noticed under the different suborders.

THE STRUCTURE OF THE HEAD. After studying the com-
position of the thorax and abdomen, where the constituent
parts of the elemental ring occur in their greatest simplicity,
we may attempt to unravel the intricate structure of the head.
We are to determine whether it is composed of one; or more,
segments, and if several, to ascertain how many, and then to
learn what parts of the typical arthromere are most largely
developed as compared with the development of similar parts
in the thorax or abdomen. In this, perhaps the most difficult
problem the entomologist has to deal with, the study of the
head of the adult insect alone is only guesswork. We must
trace its growth in the embryo. Though many writers consider
the head as consisting of but a single segment, the most emi-
nent entomologists have agreed that the head of insects is com-
posed of two or more segments. Savigny led the way to these
discoveries in transcendental entomology by stating that the
appendages of the head are but modified limbs, and homol-
ogous with the legs. This view at once gave a clue to the
complicated structure of the head. If the antennae and biting
organs are modified limbs, then there must be an elemental
segment present in some form, however slightly developed in
the mature insect, to which such limbs are attached. But the
best observers have differed as to the supposed number of such
theoretical segments. Burmeister believed that there were two
only ; Cams and Audouin thought there were three ; McLeay
and Newman four, and Straus-Durckheim recognized seven.
From the study of the semipupa of the Humble-bee (Bombus)

support of the sting; e, the support of the stylet (i). It, the amis ; O, the outlet of
the oviduct. The seventh, eighth, and ninth sternites are aborted. From Lacaze-
Duthiers.



THE STRUCTURE OF THE HEAD. 19

and several low Neuropterous forms, as the larva of Ephemera,
but chiefly the embryo of Dlplax, a dragon-fly, we have con-
cluded that there are seven such elemental segments in the
head of insects.

That there are four corresponding to the jointed appendages,
?'. c. the labium, or second maxillae, the first maxillae, the man-
dibles, and the antennae, seems indisputable. But where else
are we to look for jointed appendages in an insect's head? We
must go out of the class of Insects and study the stalk-eyed
Crustacea, such as the Lobster, where the eye is supported on a
two-jointed stalk, which has been homologized with the limbs.
While, therefore, the eyes of insects are never "stalked," as in
the Lobster and Shrimp, they are evidently developed, as in
the Crustacean, upon a -separate segment (or its rudiments),
which may be called the u ophthalmic ring," and which is, there-
fore, the fifth cephalic ring. In advance of the eyes are nor-
mally placed the three ocelli, though in the highest Insects (the
Diptera, Lepidoptera, and Hymenoptera) they appear to be
situated in the rear of the eyes.

Each of these three ocelli is situated upon a distinct piece ;
but we must consider the anterior single ocellus as in reality
formed of two, since in the immature pupa of Bombus the
anterior ocellus is differently shaped from the two posterior
ones, being transversely ovate, resulting, as I think, from the
fusion of two originally distinct ocelli, and not round like the
other two. There are, therefore, two pairs of ocelli, and hence
they grow from the rudiments of a sixth and seventh ring
respectively.

Now, since the artliropleural is the limb-bearing region in
the thorax, it must follow that this region is largely developed
in the head, to the bulk of which the sensory and digestive
organs bear so large a proportion ; and as all the parts of the
head are subordinated in their development to that of the ap-
pendages of which they form the support, it must follow logi-
cally that the larger portion of the body of the head is pleural,
a. id that the tergal, and especially the sternal, parts are either
very slightly developed, or wholly obsolete. Thus each region
of the body is characterized by the relative development of
the three parts of the arthromere. In the abdomen the upper



20



THE CLASS OF INSECTS.



(tergal) and under (sternal) surfaces are most equally devel-
oped, while the pleura! line is reduced to a minimum. In the
thorax the pleural region is much more developed, either quite
as much, or often more than the upper, or tergal portion, while
the sternal is reduced to a minimum. In the head the pleurites
form the main bulk of the region, the sternites are reduced to
a minimum, and the tergites may be identified in the occiput,
the clypeus, and labrum.



TABLE OF THE SEGMENTS or THE HEAD AND THEIR APPENDAGES,

BEGINNING WITH THE MOST ANTERIOR.*



PreoraL



Tergal,


\ .L/uurum, epipnurynx, ciy-
(- peus.


I Pleural,


C Anterior ocellus (originally
I double).


> Pleural,


Two posterior ocelli.


| Pleural,


Eyes.


I Pleural,


Antennas.


Pastoral.




} Pleural,


Mandibles.


I Pleural,


First maxillae.



(Hypothetical),
First Segment

(First Ocellary),

Second Segment
(Second Ocellary),

Third Segment
(Ophthalmic),

Fourth Segment
(Antennary),



Fifth Segment
(Mandibular),

Sixth Segment
(First Maxillary),

Seventh Segment
(Second Maxillary, or
Labial),



The Appendages. We naturally begin with the thoracic
appendages, or legs,, of which there is a pair to each ring. The
leg (Fig. 25) consists of seven joints, the basal one, the coxa, in
the Hymenoptera, Lepidoptera, and Diptera, consisting of two

* In the first column are enumerated the seven rings, or segments, composing
the head. The tergal parts (i.e. the labrum, epipharynx, and clypeus), situated in
front of the ocelli, are left out in enumerating the seven segments, as they are not
supposed by the author to belong to either of those segments.

In the first column the seven rings are named (in brackets) according to the sort of
appendages they bear. In the second column is given the part, or parts, of the ideal
segment supposed actually to exist in an insect's head; and in the third column are to
be found the names of the organs attached to their corresponding segments, beginning
with the front and going back to the base of the head.





THE APPENDAGES. 21

*

pieces, i.e. the coxa and trochantine (see Fig. 12); the tro-
c/Hinter; the femur; the tibia, and, lastly, the tarsus, which is
subdivided into from one to five joints, the latter being
the normal number. The terminal joint ends in a pair
of claws between which is a cushion-like sucker called
the pulvillus. This sucking disk enables the Fly to
walk upside down and on glass.

In the larva, the feet are short and horny, and the Fig. 25.
joints can be still distinguished. In Myriapods, each segment
of the abdomen has a pair of feet like the thoracic ones. We
must consider the three pairs of spinnerets of Spiders, which
are one to three-jointed, as homologous with the jointed limbs of
the higher insects. In the six-footed insects (Hexapoda), the
abdominal legs are deciduous, being present in the Coleopterous
grub, the Dipterous maggot, the caterpillar, and larva of the
Saw-fly, but disappearing in the pupa state. They are often,
as in most maggots, either absent, or reduced in number to the
two anal, or terminal, pair of legs ; while in the Saw-flies, there
are as many as eight pairs. These " false" or " prop-legs"
are soft and fleshy, and without articulations. At the retrac-
tile extremity is a crown of hooks, as seen in caterpillars or the
hind-legs of the larva of Chironomus (Fig. 26), in which the
prothoracic pair of legs is reduced to inarticu-
late fleshy legs like the abdominal ones.

The position of the different pairs of legs
deserves notice in connection with the principle
of " antero-posterior sj^mmetry." The fore-
legs are directed forwards like the human arms, Fi s- 26 -
but the two hinder pairs are directed backwards. In the Spiders,
three pairs of abdominal legs (spinnerets) are retained through-
out life ; in the lower Hexapods, a single pair, which is ap-
pended to the eleventh segment, is often retained, but under
a form which is rather like an antenna, than limb-like. In
some Neuropterous larvae (Phryganea, Corydalus, etc.) the
anal pair of limbs are very well marked ; they constitute the
" anal forceps " of the adult insect. They sometimes become
true, many-jointed appendages, and are then remarkably like

FIG. 25. A, coxa; B, trochantcr; C, femur; D, tibia; F, tibial spurs; E, tarsus,
divided into five tarsal joints, the fifth ending in a claw. From Sanborn.




22 THE CLASS OF INSECTS.




, as in the instance of Mantis tessettata described by
Lacaze-Dtithiers (Fig. 23). In the Cockroach these append-
ages, sometimes called "anal cerci," resemble the antennae of
the same insect. In the Lepidoptera and Hymenoptera they
do not appear to be jointed, and are greatty aborted.

The Wings. The wings of insects first appear as little soft
vascular sacs permeated by tracheae. They grow out in the
preparatory stages (Fig. 27) of the pupa from the side of the
thorax and above the insertion of the
legs, i.e. between the epimerum and
^,m tergum. During the pupa state they
are pad-like, but when the pupa skin is
thrown off they expand with air, and
in a few minutes, as in the Butterfly,
enlarge to many times their original