Partial attempts at a synthetical arrangement with regard to the larvæ of _Lepidoptera_ and the saw-flies (_Tenthredo_ L.) have been made both by Reaumur and De Geer. M. Latreille also has recently given a _Tableau méthodique et général_ of articulated animals furnished with jointed legs, considered in their first state[373]. The former of these is chiefly founded upon the number of the prolegs, and the latter upon the metamorphosis, prolegs, habits, head, and parts of the mouth, without any other notice of the configuration. Mr. Wm. MacLeay, who, though young in years, is old in science and critical acumen, has started a perfectly new hypothesis upon this subject. In the progress of his inquiries into the natural arrangement of animals, particularly of _insects_ in the _Linnean_ sense, he has been the first to observe, that the relation which organized objects bear to each other is of two kinds; one of real _affinity_, and the other only of _analogy_, or resemblance. This important distinction, upon which I shall enlarge in a future Letter, when I come to treat of Systems of Entomology, he has applied, in a way quite original, to larvæ in general, but more particularly to those of the _Coleoptera_ order. For the basis of his system he assumes a relation of analogy between the _larvæ_ of Insects that in the progress of their metamorphosis assume wings, and those that do not, which form his class _Ametabola_, so that the prototypes of the former shall be found amongst the latter[374]. But though Mr. MacLeay appears to consider the analogy between these two as _primary_, he extends it in a _secondary_ sense to the _Crustacea_, at least in several instances[375]. Upon this occasion he very judiciously remarks, that "in terming larvæ _Chilognathiformes_ or _Chilopodiformes_, it is not meant that they are _Scolopendræ_ or _Iuli_, or even near to them in affinity; but only that they are so constructed that certain analogical circumstances attending them strongly remind us of these _Ametabola_[376]." This remark you will bear in mind while I am treating of this subject. It should seem from another part of the same paragraph, that the comparison which our learned Physiologist recommends, is between the _young_ of the _Ametabola_ and the larvæ of the corresponding groups of _Coleoptera_. This must be understood to refer chiefly to the young of the _Chilopoda_ and _Chilognatha_, since they approach nearer to them in that state, having then only six legs; but the rest of the _Ametabola_ should certainly be brought to this comparison in their adult state: and even the two former orders in that state more strongly resemble numerous coleopterous larvæ, than when they are young and much shorter. I before called your attention to the remarkable circumstance that contrasts very many larvæ of Hexapod insects that become winged in their perfect state with adult _Myriapoda_: namely, that in one the progress to this state is by losing their prolegs and shortening their body; while in the other, the reverse of this takes place, numerous prolegs and additional segments being gained before they arrive at maturity[377]. As the multiplication of organs is a sign of imperfection, it may be affirmed of the former of these tribes, that their progress is towards greater perfection; while that of the other may be called a degradation. As larvæ may be regarded as a stepping-stone by which approach is made from the apterous to the winged tribes of Insects, it seems most consistent with general analogy that each should connect with the other in that state in which the resemblance is greatest. Now the _Myriapoda_ resemble larvæ, as we have just seen, most when in their _adult_ state; therefore the comparison should be between larvæ and adult _Myriapoda_.