The last of these small aberrant orders is that of the Aphaniptera, constituted for the family Pulicidæ. In their transformations, as in many other respects, they closely resemble the Diptera. Strauss Durckheim indeed said that “_la puce est un diptère sans ailes_.” Westwood, however, regards it as constituting a separate order. As indicated by the names of these orders, the structure of the wings affords extremely natural and convenient characters by which the various groups may be distinguished from one another. The mouth-parts also are very important; and, regarded from this point of view, the Insecta have been divided into two series—the Mandibulata and Haustellata, or mandibulate and suctorial groups, between which, as I have elsewhere shown,[4] the Collembola (_Podura_, _Smynthurus_, &c.) occupy an intermediate position. These two series are:— MANDIBULATA. Hymenoptera. Strepsiptera. Coleoptera. Euplexoptera. Orthoptera. Trichoptera? Thysanoptera? HAUSTELLATA. Lepidoptera. Diptera. Aphaniptera. Hemiptera. Homoptera. Again—and this is the most important from my present point of view—insects have sometimes been divided into two other series, according to the nature of their metamorphoses: “Heteromorpha,” to use the terminology of Prof. Westwood,[5] “or those in which there is no resemblance between the parent and the offspring; and Homomorpha, or those in which the larva resembles the imago, except in the absence of wings. In the former the larva is generally worm-like, of a soft and fleshy consistence, and furnished with a mouth, and often with six short legs attached in pairs to the three segments succeeding the head. In the Homomorpha, including the Orthoptera, Hemiptera, Homoptera, and certain Neuroptera, the body, legs, and antennæ are nearly similar in their form to those of the perfect insect, but the wings are wanting.” HETEROMORPHA. Hymenoptera. Strepsiptera. Coleoptera. Trichoptera. Diptera. Aphaniptera. Lepidoptera. HOMOMORPHA. Euplexoptera. Orthoptera. Hemiptera. Homoptera. Thysanoptera. Neuroptera. But though the Homomorphic insects do not pass through such striking changes of form as the Heteromorphic, and are active throughout life, still it was until within the last few years generally (though erroneously) considered, that in them, as in the Heteromorpha, the life fell into four distinct periods; those of (1) the egg, (2) the larva, characterized by the absence of wings, (3) the pupa with imperfect wings, and (4) the imago, or perfect insect. I have, however, elsewhere[6] shown that there are not, as a matter of fact, four well-marked stages, and four only, but that in many cases the process is much more gradual.