AND DEVELOPMENT OF MELOE. 181 
organized matter: we only know, of a certainty, that it is by the agency of the con- 
tractile muscles that the form of the body is rapidly altered at the period of metamorphosis, 
and that whatever is the origin or the nature of the contractile power, its evolution is 
accelerated or retarded by physical influences. Alternations of heat and cold, drought and 
“* moisture, are favourable to the changes which this power effects, and promote their occur- 
rence, as an unaltered continuance of either of the conditions mentioned retards them. 
Reaumur found that by keeping chrysalids of the common white butterfly in an ice-house, 
the changes to the perfect insect were prevented for two years; whilst by removing others 
in the depth of winter to a hot-house, he induced the appearance of the perfect insects in 
a few days. I have myself noticed similar facts in the Hymenoptera. Some larvæ of 
Anthophora, which I collected in the month of October, and preserved in a warm room 
through the winter, instead of undergoing transformation, as in their natural haunts, on 
the accession of warmth, in February and March, did not change into nymphs until some 
hot days in August, when the temperature of the apartment was greatly increased ; and 
having entered the imago state in a few days afterwards, then lapsed into perfect quiescence, 
-or sleep, as in their natural state of hybernation, and did not become active until the fol- 
lowing spring *. Thus alternations of condition are essential to the changes in growth 
and development, as to the health of the body, and to the evolution of all vital power. 
This is equally true with reference to the highest, as to the lowest of created beings; to 
the most perfect, as to the least organized ; to ourselves, as to the insect we are examining. 
Influenced by alternations of condition in the functions of respiration and nutrition, the 
muscles of the insect acquire an accumulation of contractile power before the change; 
and when the larva has attained its full size, and its further growth is arrested, the moment 
of transformation has arrived, and this power in the muscles constitutes the secondary 
and most evident means of development. Certain muscles in the insect are ranged in the 
axis of its body, in a longitudinal direction, attached to the internal surface of the tegu- 
ment in parallel series at the anterior margin of one segment, and extended to the poste- 
rior of another; and others are ranged in diagonal, or in transverse series. By the action 
of the longitudinal ones, aided by the diagonal, and operating on the whole structure, the 
main portion of the tegument is gradually separated from the worn-out external layer that 
is to be removed; and by a concentration as it were of the muscular forces in the seg- 
ments immediately behind the head, this layer is ruptured along the dorsal surface; and, 
gradually detached from the new covering beneath it, it is slipped off backwards by suc- 
cessive contractions and elongations of the segments. 
When this change takes place after the insect has acquired its full growth as a larva, 
* Since this paper was read I have repeated this observation. Some specimens of Anthophora obtained in the larva 
state on the 12th of September 1847, were preserved in a room of moderate temperature during the winter; but they 
did not change to nymphs until from the 7th to the 14th of July 1848, and then only assumed the perfect state in 
September of the same year; after which they did not throw off the last tegument until January 1849, and became 
active imagos in February. I pointed out this fact of arrested development, at a uniform high temperature, at a 
Meeting of the Entomological Society in April 1847. (See Trans. Ent. Soc. vol. v. pt. 2, 1847, p. xi.) I may men- 
tion also that five of the larvæ which were the subjects of this experiment, were of a deep yellow instead of a white 
colour, and that two of them produced male, and three female imagos, so that difference of colour has no reference to 
‘the sex of the individuals. 
VOL. XXI. 25 
