Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/888/
INSECTA. 
880 
by the cutaneous and respiratory surfaces. But 
when the changes in the internal structures are 
nearly completed, and the perfect insect is soon 
to be developed, the respiration of the pupa is 
greatly increased, and the gaseous expenditure 
of its body is augmented in the ratio of the 
volume of its respiration, which is greatest the 
nearer the period of development. Thus in the 
same insect in which the diminution of weight 
was so trifling during eight months’ quiescence 
and abstinence, it amounted in the succeeding 
fifty-one days to nearly half the original weight 
of the pupa, since the perfect insect, imme¬ 
diately after its appearance on the 24th of May, 
weighed only thirty-six grains. 
This increased activity of function is attended 
with a correspondent alteration in the general 
appearance of the pupa. In the sphinx all the 
parts of the future Imago become more and 
more apparent on the exterior of the pupa case, 
the divisions into head, thorax, and abdomen 
are more distinctly marked, the eyes, the an¬ 
tennae, and the limbs appear as if swollen and 
ready to burst their envelope, and the pupa 
gives signs of increasing activity by frequent 
and vigorous contortions of its abdominal seg¬ 
ments. The naked pupa or nymph, in which, 
as we have seen, all the parts of the body are 
free, and encased only in a very delicate mem¬ 
brane, acquires a darker colouring and a firmer 
texture, while the species which undergo their 
metamorphoses into nymphs in the water, Tri¬ 
chop ter a, the caddis-flies, acquire a power of lo¬ 
comotion as the period of their full develop¬ 
ment approaches, to enable them to creep up 
the stems of plants, and leave that medium in 
which it is impossible for them to exist as per¬ 
fect insects. 
In every instance the assumption of the per¬ 
fect state is accompanied by a slipping off of 
the external covering. Before this can be ef¬ 
fected, many Lepidoptera, like the Trichoptera, 
have first to remove themselves from the locality 
in which they have undergone their previous 
metamorphoses. When this happens to be in 
the interior of the trunks of trees, or in other 
situations from which it is difficult to escape, 
the abdominal segments of the pupa are often 
beset with minute hooks (fig. 367), similar to 
those on the feet of the larva. By means of 
these, by alternately contracting and extending 
its abdominal segments, the pupa is enabled to 
force an opening through its silken coccoon, or 
to move itself along until it has overcome the 
obstacles which might oppose its escape as a 
perfect insect.. 
The imago or perfect state.—Immediately 
after the insect has burst from the pupa case it 
suspends itself in a vertical position with its 
new organs, the wings, somewhat depending, 
and makes several powerful respiratory efforts. 
At each respiration the wings become more and 
more enlarged by the expansion and extension 
of the tracheal vessels within them, accompa¬ 
nied by the circulatory fluids. When these 
organs have acquired their full development 
the insect remains at rest for a few hours and 
gains strength, and the exterior of the body be¬ 
comes hardened and consolidated, and forms, 
what we shall presently consider, the Dermo- 
skeleton. This is what takes place in Lepidop- 
terous insects. Some of the Coleoptera, as in 
the instance of Melolontha vulgaris, the com¬ 
mon chaffer-beetle, remain for a greater length 
of time in their nidi before they come abroad 
after entering the imago state. This is also the 
case with the Humble-bees. When these in¬ 
sects first come from their cells they are exceed¬ 
ingly feeble, their bodies are soft, and covered 
with moisture, their thick coating of hairs has 
not acquired its proper colour, but is of a gray¬ 
ish white, and they are exceedingly susceptible 
of diminished warmth. They crowd every 
where among the cells, and among other bees, 
where there is most warmth. In a few hours 
this great susceptibility is diminished, and their 
bodies acquire their proper colours, but they 
do not become sufficiently strong to be capable 
of great muscular exertion, and undertake the 
labours of the nest until the following day. 
When an insect has once entered its perfect 
state, it is believed to undergo no further meta¬ 
morphosis or change of covering. But there 
exists an apparent exception to this general law 
in the Ephemeridæ, which are noted for the 
shortness of their existence in the imago state. 
When these insects have crept out of the water, 
and rid themselves of the pupa covering, and 
their wings have become expanded, they soon 
take flight, but their first movements in the air 
are performed with some difficulty, and they 
shortly alight again and throw off a very deli¬ 
cate membrane with which every part of the 
body has been covered, and then resume their 
flight with increased activity. The condition 
of the insect previously to this final change has 
been called by Mr. Curtis the pseudimago state. 
It was noticed long ago by Swammerdam, and 
has usually been thought to be peculiar to the 
Ephemeridæ, but occurs also in the Lepidoptera 
and Diptera,* but in them takes place at the 
same time with the change from the pupa state. 
Swammerdam thought the change peculiar to 
the males of the Ephemeridæ, but Mr. West- 
wood has seen it also in the females. 
Many insects, of which the Ephemeridæ and 
Bombycidæ are known examples, take no food 
in the perfect state, and exist only for a few 
hours, or at most only a few days, the business 
of life being almost entirely devoted to the pro¬ 
pagation of the species. In every instance of 
the entire abstinence of a species in the perfect 
state there is a corresponding atrophy of the 
parts of the mouth. This we shall find is the 
case in the Ephemera, in the gad-fly, Œstrus, 
and in the silk-worm moth. In the latter in¬ 
stance the parts of the mouth are simply so 
much diminished in size as to be unfitted for 
taking food; in the former they have almost 
disappeared. On the other hand, when the life 
of the imago is continued for a long period, all 
the parts of the mouth are fully developed. 
The duration of life in these species often ex¬ 
tends for many weeks, or in some even months, 
and the quantity of food taken is consequently 
greater than is taken by the larva. In those 
* Westwood’s Introduction, &c. vol. ii. p. 28.
        

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