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Psyche 6:37-49, 1891.

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PSYCHE.
A GENERAL SURVEY OF THE MODES OF DEVELOPMENT IN INSECTS AND THEIR MEANING.*
The mode of development in all of
the first series of orders from 1-IX [see Psyche v. 6, p. 131 is as a rule direct, and this necessarily unites the Thysanu- riform larva, when it is present, more
or less closely with the adult stages,
and the adults are apt to show traces of this connection in the retention of cer- tain primitive characteristics. The ab-
sence of a waist or deep constriction be- tween the thorax and the abdomen is due
to the fact that the junction with the
metathorax remains in most
adults as it
is in the larva and in Thysanura. The
mouth parts also are for biting, except
in the highly specialized Heiniptera, in which, although the suctorial character- istics of these parts are developed early, the larvae, with this exception, have
what may be called a Thysanuriform
stage. The highly specialized adults
of groups having indirect development
(Coccidae) are not exceptions to this
rule, and retain to a recognizable de-
gree the primitive form of the larvae.
The second series of orders from X-
XVI have, as a rule, more complicated
modes of development, introducing va-
rious intermediate and often extraordi-
nary stages, such as grubs, caterpillars, *From Guides for Science Teaching, No. VIII. By
Alpheus Hyatt and J. M. Arms.
etc.
Following Brauer and some other
entomologists, we have regarded these
as more or less degraded modifications
of the primitive Thysanuriform larva,
but have spoken of them collectively as
the secondary larval stages. They ap-
pear subsequently to the Thysanuriform
stage, when that is present, or between
the ovarian and pupal stages when that
is absent. The pupal stage is similar
to that of the first series of orders in all respects except that, as a rule, it is in- capable of motion, or is what is called
quiescent, and is usually more or less
protected. The complicated develop-
ment of individuals in the second series of orders has led several authors to des- ignate the first series of orders as Ame- tabola, and the second series as
Metabola.
The use of the term "ametabola," as
applied to the orders from I to IX, in-
volves an exaggeration, since it implies that they have no metamorphoses;
whereas, as pointed out by Comstock
and others, the Coccidae have a "coin-
plete" series of metamorphoses, or in-
direct development, even including a
quiescent pupal stage in the develop-
ment of the only winged form, the
male. The quiescence of the pupal
stage loses much significance in view of



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38 ~ c ~ . [March 1891.
this exception, and also when it is noted that an extra quiescent larval stage may occur in the second series of orders, as in some beetles, whose extraordinary
habits render two quiescent stages es-
sential in their development.
It is a remarkable fact that, as a rule, the larvae of the second or specialized
series of orders have the habit of feeding voraciously. In this way the larvae
store up fats- and food matters in their own bodies in preparation for the qui-
escent and helpless pupal stage, during
which they live upon these accumula-
tions, they being taken up by the cells
of the tissues and used in building' up
the organs and parts of the adult. The
pupal stage is passed, as a rule, in more or less sheltered situations, and it is
either enclosed in a special covering, a cocoon, woven by the animal, or else
protected by one acquired through the
moulting and hardening of its own cut-
icle. The difference between this last
and the ordinary process of moulting
consists in the retention of the moulted skin, the animal shrinking within it for shelter as its fatty parts are consumed, instead of casting it off altogether.
Lubbock, in his Origin and rneta-
morphoses of insects, has shown that
the inactivity of the pupa in the second series of orders is not a novel condi-
tion, but a mere prolongation of the
shorter periods of inactivity which ne-
cessarily accompany every change of
skin or moult. These facts and the ob-
vious want of any common structural
differences in the quiescent pupae, as
compared with the similar stages of ac-
tive pupae, show that quiescence must
be reckoned as a habit of resting from
active exertion during a more or less
'
prolonged period of their growth which
has been acquired by the more special-
ized forms of insects, not only generally among the members of the second series
of orders, but also by many among the
first series. The degraded larvae of in- dividuals in these specialized forms are as a rule farther removed structurally
from their own adults, than in forms
having a direct mode of development,
and the changes to be gone through be-
fore reaching the adult stage are greater and more numerous. The habits of the
animal during the pupal stage have con-
sequently changed in proportion to
these requirements from the active to
the quiescent condition.
There are other series of facts equally
important and significant. W liile the
I'hysan~~riform stage is present more or less in Coleoptera and Neuroptera,
which have the indirect mode of develop- ment, it is absent in the orders from
XI1 to XVI inclusive, having been re-
placed by the secondary larval stages in accordance with the law of acceleration
in development.
The tendency of the more speci.ilized
forms in the orders I to IX to accelerate the development of the earliest stages is shown in various ways. In the grass.
hoppers,* Mantidae, etc., the inheril
tance of the adult peculiarities of the
type affects the young at such early
*Packard's illustrations on p. 60 of his Entomology for beginners give an excellent series of one species, Caloptenus femur- rubrum




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March 1891.1 pis- YcHB. 39
stages that, as has been described above, the primitive larval 'l'hysanui-iforin stage is skipped or omitted from the develop-
ment.
In Coleoptera and in the highly spe-
cialized orders of insects (XI to XVI)
a novel and disturbing- influence appears, due to the extraordinary importance of
the functions of larval life. This period in the larger number of groups in other
classes of animals is much less variable than the adult stage, and it is really very often a mere vehicle for the record and
transmission of hereditary characters.
In some of the orders of insects, how-
ever, it is as efficient for the manifesta- tion of new modifications and adaptive
characters as the adult, and often per-
haps more variable. This is an excep-
tional rather than the usual aspect of
the larval stages, and makes the study
of insects remarkably difficult and inter- esting.
Sometimes in the orders I to IX (Coc-
cidae, Cicada), as well as more gener-
ally in X to XVI, the larvae carry the
line of development and modification a
long way outside of what can be termed
the normal or direct course, but these
deviations lead, as a rule, back again
through similar pupae to the same goal
in the imago, a typical adult insect.
Epicauta, the blister-beetle, is a good
example. Fig. 98 shows the active Thy-
sanuriform larva, and Figs. 102, 106,
107, the grub-like larva which passes
through two stages before becom-
ing the true pupa that transforms
into the imago.' These complica-
tions were probably due originally in
each type to the plastic nature of the
organism, which enabled it to fit itself to different conditions and surroundings during its passage through the younger
stages of growth. The history of para-
sites, whose loss of parts and correlative modifications are plainly adaptations to the nature of the surroundings in all
branches of the animal kingdom, shows
this to be sound reasoning. Among some
of these types there are all kinds of meta- morphoses and very complicated modes
of development, so that it is not difficult to surpass even those of insects.
One
can apply a similar nomenclature and
the same laws in explanation of the
often curious and sometimes exti-aordi-
nary metamorphoses, and these changes
are often, as in Taenia, accompanied
by corresponding acceleration and loss
of primitive stages. The curious trans-
formations of Echinodermata are plainly
adaptations of the larvae to a free life in the water before they become attached or sink to the bottom, and begin their
proper life as crawlers. In this class
there are a number of examples of accel- eration (Coinatula, Spatangoids, etc. ) . Such life-histories and those of Epi-
cauta, Sitaris and Meloe among beetles
which run out the gamut of changes
from the simplest Thysanuriform larva
through several grub stages to the qui-
escent pupa, show that the most
complicated n~etamorphoses, called hy-
permetamorphoses by entomologists,
must have arisen in response to the
changes of the surroundings. No other
hypothesis can account for the number,
variety, and novelty of these metamor-




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40 piS"YcHE. [March -1891.
phoses and their suitability to the num- ber, variety, and novelty of the changes in the surroundings and the correspond-
ing changes in habits of the larvae at
different stages of growth.
The occupation of the larval stages
by strange and curious forms, like cat-
erpillars, grubs, etc., naturally attracts attention and at first makes one wonder
at the apparent eccentricities of nature's ways. But in reality they serve to
throw a strong side light upon the nor-
mal mode of action of the laws of hered- ity, and show us that, in spite of its
enormous conservative force, heredity
is subservient to the effects of habit or use of parts.
That these secondary larval forms are
more reduced, although more special-
ized organisms than the primitive Thy-
sanuriform larvae, has already been
stated. Among Coleoptera and Neu-
roptera this is obvious whenever the
Thysanuriform and secondary adaptive
forms are present in the growth of the
same individual. No one can compare
the swollen, soft, round-bodied grubs
with the active Thysanuriform larva,
especially when occurring in the growth
of the same beetle, without realizing
that the former is due to specialization by reduction. That their structures,
although degraded by this process, are
suitable to the conditions under which
they live has been pointed out by many
writers ; notably, Graber, Riley, Lub-
bock, and Packard. This reduction
becomes still more apparent when we
regard the larvae of Diptera and the
grubs of the weevils among Coleoptera,
the latter being generally without legs, and the former also deficient in these
organs and in large part without a dif-
ferentiated head. If these or the cater- pillars or other secondary larval forms
similar to them were isolated, and their subsequent development into pupae
and adults unknown, naturalists would
no admit that they possessed close affin- ities with the adult insects of the same ' groups, and they would be considered
as more rudimentary or simpler in
structure than any Thysanuran or Thy -
sanuriform larva. In the most special-
ized forms of Coleoptera, the weevils,
the early development of a footless grub, a reduced form similar to the maggot of
the Diptera, replaces both the Thysa-
nuriform larva and also the active six-
footed grub of the normal groups of
beetles. The Insecta furnish such ap-
parently isolated examples, and, on ac-
count of the absence of intermediate
forms, it has been supposed that these
could be put in evidence against the
derivation of the orders of which they
were members from Thysanura, as has
been stated above with reference to the
saltatorial Orthoptera, but the researches of Brauer, Packard, and Lubbock, de-
monstrating that the secondary larval
stages, grubs, maggots, etc., are modi-
fications of the Thysanuriform larval
stages, show that this use of them can-
not be admitted. If this be granted, it
becomes possible to account for the phe- nomena as follows. The modified, and
adaptive, larval characters of the grubs, caterpillars, etc., havhg become fixed
in the organization of such groups as




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March iSg1.1
PSYCHE.
the weevils among Coleoptera, and in
some whole orders, as in the Lepidop-
tera and Diptera, have been inherited
at such early stages in accordance with
the law of acceleration in development
that they have replaced the useless Thy- sanuriform stage. In other words, the
absence of this primitive larval stage in the young of many specialized forms of
insects now living is due to the tendency to earlier inheritance of the later ac-
quired, adaptive characters of the secon- dary larval forms.
It is very important for these consid-
erations to notice that after the insects possessing the indirect modes of devel-
opment have passed through their re-
ductive secondary larval stages, they
return to the more normal or direct
mode of development in the pupa. In
doing this, they clearly illustrate the
exceptional and adaptive nature of their deviations from the direct mode during
the larvd stages, and show that this re- sumption of the older beaten path
marked out by heredity is essential in
order that a typical hexapod form may
be evolved in the adult stage. The
pupa is always a six-legged form, with
the legs more or less developed, and
being common to all insects, whether
quiescent or active, is really a part of the direct mode of development wher-
ever it occurs. It is as universal and
essential as are the typical ovarian and adult stages. Indirect development is,
therefore, composite. It is first a devia- tion in the larva from the direct mode,
and then a return in the pupa of the
direct mode, and this return necessarily brings the organism back again into the
normal line of evolutionary changes,
and the normal form of insect is the re- sult of this return and the resumption
of prog&ssive specialization.
The reverse of this process, i. e.
when direct development is not re-
sumed, is shown in the case of parasites like the female of Stylops.
If it be true that the stages of develop- ment in individuals are abbreviated rec- oi-ds of the modifications undergone by
the group during its evolution in time,
and that as a rule the characteristics oi' adults of the more generalized or pi-imi- five forms of any order, or even of
smaller divisions, in all groups of the
animal kingdom, show a tendency to
occur in the young of more specialized
forms of the same group 01- division, it follows, that in each natural group the
specialized forms have been evolved
from the generalized forms. This ten-
dency to accelerate and abbreviate the
record preserved by heredity in the
growth and development of each indi-
vidual can be understood if one imagines a series of forms evolving in time. First, the representatives of the simple, primi- tive ancestor; then one form after
another coming into being successively
would each introduce some novel modi-
fications, according to its place in time and the structural series. These modi-
fications being inherited at earlier stages in descendants than those in which they
originated in the ancestral forms, would crowd upon the characteristics already
fixed by heredity in the growth of the
young. By and by, as characteristics




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accumulated, it would become not only
inconvenient to repeat all the character- istics of its ancestors, but it wotild be a physical impossibility for any individual to reproduce them all in the &me suc-
cession in which they had arisen ; life
would not be long enough nor vital pow-
ers strong enough to accomplish such a
process. Nature provides for such
emergencies by a law of replacement ;
and as stated above, when a part or
characteristic becomes useless, if it
stand in the way of the development of
other parts or other characteristics of the same part, it is replaced to a greater or less degree by the newer and more useful modifications. This is the rule so far
as relates to an ordinary normal series
of forms when such a series can be
traced with abundant materials through
a sufficiently long period of geologic
time, as has been repeatedly shown by
Cope and one of the authors. Made
confident by such experiences we do
not hesitate to apply it to the insects
where positive evidence of this sort is
not yet forthcoming.
If this be correct, it is evident for ex- ample that the sucking-tube and other
correlative internal modifications origi- nated in the pupal or adult stages of the primitive Hemipteron, then became
fixed in the organization of the order,
and are now inherited at an early age,
having replaced or driven out the ances- tral, primitive, perhaps Thysanuriform
mouth parts from the larval stage.
The
assumption that the sucking mouth parts
originated in the pupal or adult stages
is considered probable, because, al-
though there are many exceptions, char-
acteristics usually originate in the later stages in other branches of the animal
kingdom. In Lepidoptera and Diptera,
which resemble the Hemiptera in hav-
ing the highly modified mouth parts
with a tubular arrangement, these char-
ac.teristic peculiarities are confined to the later stages of development, and are not found in their larvae. The larvae
of Hemiptera are also decidedly Thy-
samn-iform, and that they originated
from a modified Thysanuroid form
having biting mouth parts in the larvae
and sucking mouth parts in the later
stages, seems to be indicated by this
fact. We have already seen in such ex-
amples as the locusts, etc., that an earlier development in the inheritance of the
characters of adults may effectually ob- literate the Thysanuriform larva, and
in the Coleoptera, Neuroptera, etc., that it is the earlier inheritance of the sec- ondary larval characteristic which ac-
complishes this result. In no case do
the pupal or adult characteristics become accelerated in development so as to re-
place the larval stage in the second se- ries of orders except in parasites such