Viviparity

Diploptera punctata is the only known viviparous species of cockroach. Its ootheca contains about a dozen small eggs and has an incomplete oothecal membrane (Roth and Hahn, 1964). Initially the eggs lack sufficient yolk and water to complete development (Roth and Willis, 1955a), but embryos ingest water and nutritive material synthesized and transported by the walls of the brood sac at a rate paralleling embryonic growth (Stay and Coop, 1973, 1974; Ingram et al., 1977). The brood sac "milk" is composed of about 45% protein, 5% free amino acids, 25% carbohydrates, and 16-22% lipids. The milk proteins are encoded by a multigene family that arose via the modification of genes preexisting in ovoviviparous species (Williford et al., 2004). Embryos begin oral intake of the milk just after closure of their dorsal body wall and continue until shortly before partition. The ultimate source of nutrition for the embryos is the food intake of the mother; females normally double their body weight during gestation, and the embryos of starved females die.

Diploptera nymphs are large and well developed when they emerge, requiring fewer molts to adulthood than any studied cockroach. Egg fresh weight increases more than 73 times during gestation (Table 7.2) (Roth and Willis, 1955a), while the fresh weight of the ovoviviparous species N. cinerea doubles. In the latter, the weight increase is correlated solely with the absorption of water; solids are slowly lost until partition (Roth and Willis, 1955c). Neonates of D. punctata are at least twice the size of those of N. cinerea (see Fig. 3 in Roth and Hahn, 1964), yet adults of the latter are considerably larger than field-collected adults of D. punctata (approximately 27 mm and 17 mm in length, respectively—Cochran, 1983a; WJB, unpubl. data). Diploptera females have three or four post-embryonic instars, compared with the usual seven to 13 in a sample of 11 other species of Blattaria (Willis et al., 1958). This suggests that D. punctata completes a substantial proportion of its juvenile development as an embryo, with a corresponding decrease in the duration of post-embryonic development. During embryogenesis,

Fig. 7.5 Oothecae of two Panesthiinae. (A) Thin, membranous, incomplete oothecal case of Panesthia cribrata (ovovi-viparity A). (B) Massed eggs of Geoscapheus dilatatus, a species that lacks an oothecal case (ovoviviparity B). Photos courtesy of Harley Rose.

Table 7.2. Changes in wet weight,water,and solids of cockroach eggs during embryogenesis (Roth and Willis, 1955a).

Factors by which initial weights change, per egg

Table 7.2. Changes in wet weight,water,and solids of cockroach eggs during embryogenesis (Roth and Willis, 1955a).

Factors by which initial weights change, per egg

Species

Wet weight

Water

Solids

Blatta orientalis

1.21

1.35

0.96

Blattella vaga

1.12

1.32

0.81

Blattella germanica

1.21

1.49

0.74

Nauphoeta cinerea

2.11

4.62

0.81

Diploptera punctata

73.47

85.80

49.28

closure of the dorsal body wall occurs at 19% of gestation, after which the embryos begin feeding on maternal secretions (Stay and Coop, 1973). Dorsal closure occurs at 46% of gestation time in R. maderae (Aiouaz, 1974), at 50% of gestation in N. cinerea (Imboden et al., 1978), and at 56% of gestation in P. americana (Lenoir-Rousseaux and Lender, 1970). Gestation of D. punctata embryos takes 63 days at 27°C (Stay and Coop, 1973); nymphs require just 43 to 52 days to become adults (Willis et al., 1958).

As might be expected of a group of embryos competing for food in a limited space, fewer eggs incubated by the mother results in larger nymphs. This was shown experimentally by Roth and Hahn (1964), who reduced the size of the litter in D. punctata by surgically removing one of the ovaries. Neonates in these broods were larger than those of control families, presumably because of the greater amount of nutritive material made available to the fewer developing embryos. In ovoviviparous N. cinerea, R. maderae, and Eublaberus posticus, however, the size of nymphs remains constant regardless of the number of incubated eggs (Roth and Hahn, 1964; Darlington, 1970). Nymphs within the same ootheca of D. punctata also can differ considerably in size depending on their position during development; embryos that have poor contact with the wall of the brood sac have less ready access to the nutritive secretion provided by the mother (Roth and Hahn, 1964). Neonate size, in turn, influences the number of stadia required to reach adulthood, the developmental response of individuals to their social environment, final adult size, and male sexual performance (Woodhead, 1984; Holbrook and Schal, 2004).

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