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phallotreme aedeagus c

FIGURE 3.33. Origin and development of the phallic organs. (A) Primary phallic lobes; (B) paired penes of Ephemeroptera; and (C) formation of the aedeagus. [Reproduced by permission of the Smithsonian Institution Press from Smithsonian Miscellaneous Collections, Volume 135, "A revised interpretation of the external reproductive organs of male insects," Number 6, December 3, 1957, 60 pages, by R. E. Snodgrass: Figures 1A-C, page 3. Washington, D.C., 1958, Smithsonian Institution.]

groove that is enlarged proximally into the basal bulb. When an insect stings the poison gland releases its fluid into the basal bulb. The poison is caused to flow along the groove by back and forth movements of the lancets (modified first gonapophyses).

Male. "The great structural diversity in the male genitalia of insects is the delight of taxonomists, the despair of morphologists," wrote Snodgrass (1957, p. 11). Paralleling this diversity of structure is a mass of taxonomic terms (see Tuxen, 1970). The genitalia are composed of two sets of structures. First, there are basic structures that, though varied in form, are common to all insects, and second, there are structures that are peculiar to a group or even a species. Space does not permit description of the latter, or even the variation that occurs within the former, set of structures. The following paragraphs will, therefore, be limited to a discussion of the basic form of the genitalia.

Comparative study indicates that in all insects the basic genitalia are derived from a pair of primary phallic lobes, ectodermal outgrowths belonging to the tenth segment (Figure 3.33A). However, only in Ephemeroptera do they remain as separate lobes, through the posterior tips of which open the gonopores (Figure 3.33B). In Zygentoma the lobes meet in the midline to form a short, tubular structure, the "penis" The latter is, in fact, a misnomer, since this structure is not an intromittent organ. In Odonata secondary copulatory structures are developed on the ventral surface of the anterior abdominal segments (see Chapter 6, Section 3), and the genitalia on the tenth abdominal segment are much reduced. In all of the remaining orders the primary lobes are each divided into two secondary lobes, the phallomeres. Between the median pair, mesomeres, the ectoderm invaginates to form the ejaculatory duct. The outer pair, parameres, elongate and develop into clasping organs. The mesomeres unite medially to form a tubular intromittent organ, the aedeagus, whose inner passage is termed the endophallus (Figure 3.33C). The distal opening of the endophallus is the phallotreme. Occasionally the parameres and aedeagus are united basally, this region being known as the phallobase. Usually, however, the parameres are placed laterally on the ninth segment.

Two assumptions that are generally made concerning the male genitalia are (1) that they are modified limb appendages and (2) that they belong to the ninth abdominal segment. Snodgrass (1957) questioned the validity of both assumptions. He presented a convincing argument against assumption (1) and suggested, instead, that the genitalia are derived from

88 primitive paired penes. This view contrasts, of course, with that of Kukalova-Peck (1991)

who identified the various segments of the ancestral limb that make up the genitalia. Ac-

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