We have arrived, then, at our Leptus, with a head bearing two pairs of jaws. The spiders and mites do not advance beyond this stage. But in the true insects and Myriopods, we have the addition of special sense organs, the antennae, and another pair of appendages, the labial palpi. It is evident that in the ancestor of these two groups the first pair of appendages became early adapted for purely sensory purposes, and were naturally projected far in advance of the mouth, forming the antennae. Before considering the changes from the mandibulate form of insects to those with mouth parts adapted for piercing and sucking, we must endeavor to learn how far it was possible for the caterpillar or maggot to become evolved from the Leptus-like larvae of the Neuroptera, Orthoptera, Hemiptera and most Coleoptera. I may quote from a previous article[28] a few words in relation to two kinds of larvae most prevalent among insects. "There are two forms of insectean larvae which are pretty constant. One we call _leptiform_, from its general resemblance to the larvae of the mites (Leptus). The larvae of all the Neuroptera, except those of the Phryganeidae and Panorpidae (which are cylindrical and resemble caterpillars), are more or less leptiform, _i. e._, have a flattened or oval body, with large thoracic legs. Such are the larvae of the Orthoptera and Hemiptera, and the Coleoptera (except the Curculionidae; possibly the Cerambycidae and Buprestidae, which approach the maggot-like form of the larvae of weevils). On the other hand, taking the caterpillar or bee larva, with their cylindrical, fleshy bodies, in most respects typical of larval forms of the Hymenoptera, Lepidoptera and Diptera, as the type of the _cruciform_ larva, etc. * * * The larvae of the earliest insects were probably leptiform, and the cruciform condition is consequently an acquired one, as suggested by Fritz Mueller."[29] It seems that these two sorts of larvae had also been distinguished by Dr. Brauer in the article already referred to, with which, however, the writer was unacquainted at the time of writing the above quoted article. The similar views presented may seem to indicate that they are founded in nature. Dr. Brauer, after remarking that the Podurids seemed to fulfil Haeckel's idea of what were the most primitive insects, and noticing how closely they resemble the larvae of Myriopods, says, "specially interesting are those forms among the Poduridae which are described as Campodea and Japyx, since the larvae of a great number of insects may be traced back to them"; but he adds, and with this view we are unable to agree, "while others, the caterpillar-like forms (Raupenform), resulted from them by a retrograde process, and also the still lower maggot-like forms. While on the one hand Campodea, with its abdominal feet, and the larva of Lithobius are related, so on the other the Lepismatidae, which are very near the Blattariae, are nearly