We are here as little concerned with a mere suppression of organs through arrested development as in the case of the Cirripedia; the transformation and reconstruction of the whole body goes even much further than in these Crustacea, although not so conspicuous externally. Where do we elsewhere find insects having the head inside a cavity of the body (sectorial head of the _Muscidæ_), and of which the foremost segment--the physiological representative of the head--consists entirely of the coalesced antennæ and pairs of maxillæ? The incongruences in the form-relationships are, however, exceedingly numerous in the case of the Diptera, and a special treatise would be necessary to discuss them thoroughly. I may here mention only one case, because the inequality shows itself in this instance in a quite opposite sense. Gerstäcker, who is certainly a competent entomologist, divides the Diptera into three tribes, viz. the _Diptera genuina_, the _Pupipara_, and the _Aphaniptera_. The latter, the fleas, possess in their divided thoracic segments and in their jointed labial appendages characters so widely divergent from those of the true Diptera and of the _Pupipara_ that Latreille and the English zoologists have separated them entirely from the Diptera and have raised them into a separate order.[210] Those who do not agree in this arrangement, but with Gerstäcker include the fleas under the Diptera, will nevertheless admit that the morphological divergence between the _Aphaniptera_ and the two other tribes is far greater than that which exists between the latter. Now the larvæ of the fleas are completely similar in structure to those of the gnat-type, since they possess a corneous head with the typical mouth parts and antennæ and a 13-segmented body devoid of legs. Were we only acquainted with the larvæ of the fleas we should rank them with the true Diptera under the sub-order _Nemocera_. On first finding such a larva we should expect to see emerge from the pupa a small gnat. While the imagines of the _Nemocera_ and _Aphaniptera_ thus show but a very remote form-relationship their larvæ are very closely allied. Can any one doubt that in this case it is not the larva but the imago which has diverged to the greatest extent? Have not the fleas moreover become adapted to conditions of life widely different from those of all other Diptera, whilst their larvæ do not differ in this respect from many other Dipterous larvæ?