64: MR. J. LUBBOCK ON THE DEVELOPMENT OF 
having introduced themselves into the cells of certain species of Hymenoptera, they 
undergo a retrograde metamorphosis, lose their legs, and emerge as grubs, not altogether 
unlike those whose places they have usurped. 
When an insect is destined throughout life to exist in the same manner and to use the 
same food, then it leaves the egg with the principal organs constituted in the same manner 
as in the imago. 
Thus, then, we may lay it down as a general rule, that the form of an insect at its birth 
is à function of the life which it is to lead, and of the group to which it belongs; and that 
insects are generally born at a period in their development as early as is consistent with 
the life to which they are destined. 
There are doubtless many apparent exceptions. Thus, among the Diptera, lignivorous 
larvae, like those of some Tipulidze, terricolous larvee, like those of others, and predaceous 
larve, as in Syrphus, are all fleshy, apodal grubs. A better acquaintance with their 
habits would probably explain the anomaly: the lignivorous Tipulide, for instance, 
principally affect decaying wood; and in the case of Syrphus, we may already see that 
the condition of the young larva, surrounded by crowds of helpless Aphides, differs 
in reality but little from that of an internal parasite. 
But while the preceding has reference to the degree of change which takes place after 
birth, the manner also must be considered. In some insects the development is slow and 
gradual, while in others the growth is effected without any material change of form, and 
the metamorphosis, if not sudden, is at least comparatively rapid. 
Now, as long as any organ continues to be in a state of functional activity, the changes 
taking place in it must be slow and gradual; when therefore the metamorphoses are 
rapid, and are accompanied by only one or two changes of skin, we at once see that 
they necessitate a period of quiescence. When, moreover, as in the Lepidoptera, a mouth, 
originally mandibulate, is destined to become suctorial, any such gradual change would 
be inconvenient or impossible; the insect might starve in the meantime. ere, there- 
fore, it becomes desirable that the change should be rapid. "The Hemiptera, indeed, 
which are suctorial, are also active through life; but in them the mouth of the larva is 
constituted in the same manner as that of the imago. 
Again, when extensive changes have to take place in the muscular system, a period of 
enforced quiescence is the result. Some of the aquatic Diptera offer, indeed, an apparent 
exception; but in these the muscles which move the pupa are those of the abdomen, 
while those of the imago are situated in the thorax. 
It may still be asked, to what general cause can we ascribe these metamorphoses ? why 
should not insects remain in the egg until their generic and specific characters are fully 
developed ? Omitting for the present all consideration of terrestrial animals, we cannot 
but be struck by the poverty of the freshwater fauna when compared with that of the 
ocean. The Mollusca are far less numerous and less varied ; of the Fish the same may 
be said; compared with those of the sea, our freshwater Bryozoa are quite insignificant 
Ke numbers; the Hydrozoa are represented by only two genera. Among Crustacea, the 
Podophthalms have in this country but one freshwater species, the Isopods one, the 
Amphipods very few; Entomostraca, indeed, are well represented, but Cirrhipedes are 
