AND DEVELOPMENT OF MELOE. 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 Meloe, 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 minutiae 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 of the 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, 

 Meloe, 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 



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