AND DEVELOPMENT OF MELOË. 175 
exemplify them, together with the views of Schwann on the formation of tissues, in the 
Anatomy of Meloe, and to apply the principles on which they are based to the functions 
also of animated existence, in illustration of their dependence on special structure. 
It is the great principle of gradational development which operates so markedly in the 
organization and habits of many of the Articulata, and which causes them, as we have 
already seen in Stylops and Meloé, to differ so greatly in every respect in their young and in 
their adult states. In each of these, the general conformation of body, and of each particular 
organ, seems to have reference to some speciality of structure or of habit; but,—owing to 
our imperfect knowledge,—as who will presume to say, in denial of this view, that he is cog- 
nizant of all the facts in the natural history of even one species of animal ?—the object or 
applicability of every variation of structure is not always readily traceable in its details of 
colour, of armature, of size, or even in the minutis of form, although invariably evident in 
general design. We have seen this in the structure of the mandible, in the condition of the 
eye, in the size and power of the limbs, in the peculiarities of their tarsi (fig. 12), in the acute- 
ness of the physical senses, and in the vivacity of the movements of the young Meloe in its 
incipient parasitism ; and also like, but less needed, and consequently less marked conditions 
in Stylops. In both we have seen that gradational changes begin to be effected in the organi- 
zation ofthe animal immediately the physical conditions in which it is placed are altered; and 
that these changes commence in its tegument. The Stylops larva, covered with its arma- 
ture of spines, penetrates insidiously into the body of the Bee, and, engorged with nutritious 
and stimulating juices, increases rapidly in bulk, casts its embryo covering, and from an 
active becomes an almost quiescent being. Its elongated limbs are atrophied and reduced 
to mere tubercles. The spines that arm the margins of its segments,—doubtless, designed 
by creative Omnipotence to aid it in forcing its way into the body of the bee-larva,—as the 
spines on the pupa-case of the Cossus assist that insect in its transit to the outlet of its 
burrow in the trunk of the Willow, and enable it to force its way through its strong 
silken cocoon, preparatory to its liberation as a Moth,—then become utterly useless to 
the young Stylops, are entirely thrown off at its change of tegument. In like manner, 
Meloé, most active immediately after it has left the egg, and when designed to attach 
itself to the irritable Bee for conveyance to its nest, gradually becomes, after it is lo- 
cated and nourished there, the heavy apodal pseudo-larva. The structure of its tegument 
then undergoes considerable change. The forces of growth in this tissue, centred in the 
nuclei of its cells, and the repeated division and development of these into constituent 
producing portions of the whole, seem gradually to become less and less energetic at each 
change of tegument, the intervals of which are progressively extended. When reproduc- 
tion in these constituents is long retarded, throughout the whole or chief portion of them, 
their arrest seems to limit the entire bulk and form of the being in that stage of its exis 
ence, and new series of changes are induced. But when growth proceeds less rapidly in 
some of them than in others, the form of the entire body, or of some particular region of it, 
is changed. The tegument of the pseudo-larva, and that which the adult larva throws off 
on assuming this condition, afford ample demonstration of this view. The body of the larva, 
altered from that of the slender, agile little being, with elongated limbs, and long caudal 
styles, as when it left the egg, to the heavy, fat, convex grub (fig. 13), has been changed in 
242 
