Functional Significance

External pits, "bowls," or depressions function as reservoirs for the tergal secretion oozing up from underlying glandular cells (Roth, 1969; Brossut and Roth, 1977; Sreng, 1979b). In some instances, drops of liquid can be seen forming at the opening of the gland as the female feeds (e.g., R. maderae—Roth and Barth, 1967). The secretion produced by the tergal glands is a mixture of short-range volatile and non-volatile fractions, the latter including protein, lipids, and carbohydrates (Brossut et al., 1975; Korchi et al., 1999). The best-studied, that of B. germanica, is a complex synergistic mixture of polysac-charides, 17 amino acids, and lipids, including lecithin and cholesterol. Maltose, known from baiting studies to be a potent phagostimulant for the species, is one of the primary sugars (Kugimiya et al., 2003; Nojima et al., 1999a, 1999b). There is little relationship between response to the secretion and sexual receptivity. Both sexes and all stages are attracted (Nojima et al., 1999b). Because tergal secretions exploit a female's underlying motivation to feed, they can be classified as "sensory traps" (Eberhard, 1996). They mimic stimuli that females have evolved, under natural selection, for use in other contexts.

It is uncertain to what degree tergal secretions provide a nutritional boost to grazing females. The behavior is most often described as "licking" or "palpating," but the action of the female's mandibles and the manner in which she presses her mouthparts against the male's gland indicate that she actually eats the secretion. The male typically lets her feed 3 -7 sec before attempting to make genitalic connection (Roth and Willis, 1952a; Barth, 1964; Roth, 1969). Females of Eurycotis floridana may graze for nearly a minute, longer than any other studied species (Barth, 1968b). Feeding may also be "quite prolonged" in Peri-planeta spp., with the female vigorously biting the tergite. The male gland in Rhyparobia maderae can be extensively scarred (Simon and Barth, 1977b), attesting to female enthusiasm for the fare. Roth (1967c) suggested that in species with very deep, well-developed tergal glands located near the base of the male's wings, females may feed on tergal secretions during the entire period of copulation, that is, they may not rotate off the male's back into the opposed position. The extent to which tergal glands provide females with a significant source of nourishment is in need of examination, particularly in species with large glandular reservoirs. In many insects with courtship feeding the food gift provides no significant nutritional benefit to the female (Vahed, 1998). The amount of secretion ingested by B. germanica does seem negligible. On the other hand a female may feed on the tergal secretion of the male 20 times in a half hour without resultant copulation (Table 6.2), and courtship activities can deplete the gland (Kugimiya et al., 2003).

Blattella germanica is a good example of the concept that in species utilizing sensory traps, males are selected to exaggerate the attractiveness of the signal while minimizing its cost (Christy, 1995). The German cockroach has double pouches on the seventh and eighth tergites, with the ducts of underlying secretory cells leading to the lumen of the pouch (Roth, 1969). During courtship, the female feeds on the secretions in the cavities on the eighth tergite. After 2-5 sec, the male slightly extends his abdomen, causing the female to switch her feeding activities to the gland on the seventh tergite, triggering genitalic extension on the part of the male. The female can contact the tergal secretions with her palps, but the cuticular openings of the glands are too small to permit entry of the mandibles and allow a good bite. She plugs her paraglos-sae into the cavities and ingests the tiny amount of glandular material that sticks to them. The forced lingering as she repeatedly tries to access the secretions keeps her positioned long enough for a copulatory attempt on the part of the male (Nojima et al., 1999b). The tergal glands in B. germanica are akin to cookie jars that allow for the insertion of your fingers but not the entire hand. The design encourages continued female presence, but frugally dispenses what is presumably a costly male investment. Males of other species may take a more direct approach to "encouraging" females to maintain their position. In a number of Ischnoptera spp., the tergal gland is flanked by a pair of large, heavily sclerotized claws, each of which has four stout, articulated setae forming the "fingers." When the female is feeding on the tergal gland she must place her head between these claws "and probably applies pressure to the articulated setae" (Roth, 1969, Figs. 47-53; Brossut and Roth, 1977, Figs. 18-19). These structures, however, are quite formidable for simple mechanorecep-tors, and may function in restraining the female rather than for just signaling her presence.

Because tergal secretions are sampled by the female prior to accepting a male or his sperm, they may provide a basis for evaluating his genetic quality, physiological condition (Kugimiya et al., 2003), or in some species, his

Table 6.2. Summary of sexual behavior of 10 pairs of Blattella germanica observed for 30 min; from Roth and Willis (1952a), LMR's first published study on cockroaches.

Pair number

Behavior of _

cockroaches 1 2 34 5 6 7 8 910

Table 6.2. Summary of sexual behavior of 10 pairs of Blattella germanica observed for 30 min; from Roth and Willis (1952a), LMR's first published study on cockroaches.

Pair number

Behavior of _

cockroaches 1 2 34 5 6 7 8 910

Number of times male courted female1

20

44

4

14

27

17

37

48

33

17

Number of times female fed on tergal gland2

10

19

1

0

2

9

10

9

20

3

Time (sec) male in courtship position

679

1385

59

169

576

698

997

1106

916

576

Copulation successful?

+

1Courting defined as the male elevating and holding his wings and tegmina at a 45 to 90 degree angle.

2This figure also indicates the number of times the male tried to engage the female's genitalia, which almost invariably occurs after she has fed on the tergal gland for several seconds.

1Courting defined as the male elevating and holding his wings and tegmina at a 45 to 90 degree angle.

2This figure also indicates the number of times the male tried to engage the female's genitalia, which almost invariably occurs after she has fed on the tergal gland for several seconds.

ability to provide a hearty postnuptial gift in the form of uric acid (discussed below). Oligosaccharides in the tergal secretion of B. germanica do vary individually and daily (unpublished data in Kugimiya et al., 2003). Perhaps repeated tasting by the female (Table 6.2), then, is an evaluation process. Alternatively, females may need to exceed a certain threshold of contact with or ingestion of the tergal secretion before accepting genitalic engagement (Gorton et al., 1983). Finally, she may simply be trying to maximize her nutritional intake. Repeated instances of a female applying her mouthparts to a male tergal gland but leaving without copulation is particularly prevalent in starved females (Roth, 1964b). Nojima et al. (1999a) suggested that tergal secretions may be indirect nutritional investment in progeny, but the nutritional value to the female and her offspring remains to be demonstrated.

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