Title: Aquatic insects of California, with keys to North American genera and California species
Identifier: aquaticinse00usin (find matches)
Year: 1956 (1950s)
Authors: Usinger, Robert L. (Robert Leslie), 1912-1968
Subjects: Aquatic insects; Insects
Publisher: Berkeley, University of California Press
Contributing Library: University of Connecticut Libraries
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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69 Bentmck: Structure and Classification
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Fig. 1:1. An adult Plecoptera, Pteronorcys californica Newport, illustrating general insect anatomy (Essig, 1942). at all or only tiny vestiges thereof and are referred to as apterous. The main wing veins develop along the course of tracheae present in the developing wing of the immature insect. The wing venation varies greatly from order to order but certain similarities in basic pattern lend support to the theory that all the diverse patterns have been derived from a single type of primitive venation. In figure 1:4 is shown the venation pattern which students generally regard as the archetype pattern from which the venation of modern insects has been derived. The names of the veins are indicated with the standard abreviations used for the Comstock-Needham system of wing-vein nomenclature. In wings of modern insects the precosta (Pc) is lacking, and the anterior fork of the media (MA) is usually absent. The remaining veins may undergo modification by fusion of adjacent veins or by partial or complete suppression of one or more veins. In some orders the pattern is greatly complicated by the addition of secondary veins, and in other orders there are forms in which practically all venation has been lost. The abdomen typically consists of eleven segments the last of which bears the cerci. The Collembolla are a notable exception in possessing never more than six abdominal segments, either in the embryo or the adult. Various numbers of the terminal segments are involved in the formation of the external genital apparatus and ovipositor or egg-laying structure. Typically there is a pair of spiracles on each of the first eight abdominal segments, but some or all of these may be absent in some insect. Likewise the number of abdominal segments may be actually or apparently reduced or the first segment may be incorporated into the thoracic region. INTERNAL ANATOMY Just within the body wall and attached to it are the muscles. The alimentary canal, divisible into fore-, mid-, and hind-gut, occupies the center of the body and extends from one end to the other. The fore-gut is further divided into the pharynx, esophagus, crop, and proventriculus. The mid-gut represents the stom-

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