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 antennæ, 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 antennæ.
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 larvæ 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 larvæ most prevalent among insects. "There are two forms of insectean larvæ which are pretty constant. One we call leptiform, from its general resemblance to the larvæ of the mites (Leptus). The larvæ of all the Neuroptera, except those of the Phryganeidæ and Panorpidæ (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 larvæ of the Orthoptera and Hemiptera, and the Coleoptera (except the Curculionidæ; possibly the Cerambycidæ and Buprestidæ, which approach the maggot-like form of the larvæ 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 larvæ of the earliest insects were probably leptiform, and the cruciform condition is consequently an acquired one, as suggested by Fritz Müller."[29] It seems that these two sorts of larvæ 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 Hæckel's idea of what were the most primitive insects, and noticing how closely they resemble the larvæ of Myriopods, says, "specially interesting are those forms among the Poduridæ which are described as Campodea and Japyx, since the larvæ 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 Lepismatidæ, which are very near the Blattariæ, are nearly related to the Myriopods, since their abdominal segments often bear appendages (Machilis). The Campodea-form appears in most of the Pseudoneuroptera [Libellulids, Ephemerids, Perlids, Psocids and Termes], Orthoptera, Coleoptera, Neuroptera, perhaps modified in the Strepsiptera [Stylops and Xenos] and Coccidæ in their first stage of development, and indeed in many of these at their first moult." Farther on he says, "A larger part of the most highly developed insects assume another larva-form, which appears not only as a later acquisition, through accommodation with certain definite relations, but also arises as such before our eyes. The larvæ of butterflies and moths, of saw flies and Panorpæ, show the form most distinctly, and I call this the caterpillar form (Raupenform). That this is not the primitive form, but one later acquired, we see in the beetles. The larvæ of Meloë and Sitaris in their fully grown condition possess the caterpillar form, but the new born larvæ of these genera show the Campodea form. The last form is lost as soon as the larva begins its parasitic mode of life. * * * The larger part of the beetles, the Neuroptera in part, the bees and flies (the last with the most degraded maggot form) possess larvæ of this second form." He considers that the caterpillar form is a degraded Campodea form, the result of its stationary life in plants or in wood.
Pl 2. EXAMPLES OF LEPTIFORM LARVÆ.
Explanation of Plate 2. Figure 1, different forms of Leptus; 2, Diplax; 3, Coccinella larva; 4, Cicada larva; 5, Cicindela larva; 6, Ant Lion; 7, Calligrapha larva; 8, Aphis larva; 9, Hemerobius larva; 10, Glyrinua larva; 11, Carabid larva; 12, Meloë larva.
Pl 3. EXAMPLES OF ERUCIFORM LARVÆ.
Explanation of Plate 3. Figure 1. Panorpa larva; 2, Phryganea larva; 3, Weevil larva; 4, third larva of Meloë; 5, Chionea larva; 6, Carpet Worm; 7, Phora larva; 8, Wheat Caterpillar; 9, Sphinx Caterpillar; 10, Acronycta? larva; 11, Saw Fly larva; 12, Abia Saw Fly larva; 13, Halictus larva; 14, Andrena larva.
204. Tipula Larva.