External Egg Retention

In cockroaches displaying oviparity type B, the egg cases are carried externally for the entire period of embryogenesis with the end of the ootheca closely pressed to the vestibular tissues of the female's genital cavity. The proximal end of the egg case is permeable, allowing for transport of water from the female to the developing eggs (Roth and Willis, 1955b, 1955c; Willis et al., 1958). Recently, Mullins et al. (2002) injected radiolabeled water into female Blattella germanica carrying egg cases. The water was detected moving from the female to the proximal end of her ootheca, then spreading throughout the egg case following a concentration gradient (Fig. 7.3). A variety of water-soluble materials were also transferred across the female-ootheca divide, including glucose, leucine, glycine, and formate. Preliminary experiments of these authors indicate that the labeled materials also can be detected in nymphs after hatch. Scanning electron microscopy and the use of fluorescent stains pinpointed the structural basis of flow into the ootheca (Fig. 7.4). Small pores completely penetrating the oothecal covering are

Fig. 7.3 Distribution of radiolabel in oothecae attached to Blattella germanica females at four time intervals after injection of 3H2O into the females. See original paper for sample sizes and variation. After Mullins et al. (2002), with permission from The Journal of Experimental Biology. Image courtesy of Donald and June Mullins.

Fig. 7.3 Distribution of radiolabel in oothecae attached to Blattella germanica females at four time intervals after injection of 3H2O into the females. See original paper for sample sizes and variation. After Mullins et al. (2002), with permission from The Journal of Experimental Biology. Image courtesy of Donald and June Mullins.

Fig. 7.4 Scanning electron microscopy images of Blattella germanica oothecae, demonstrating the morphological basis of their permeability. (A) Proximal end of an ootheca showing the "escutcheon-shaped" vaginal imprint (arrow). (B) Magnification of the ventro-lateral escutcheon region; arrow indicates the pore field area. (C) Magnification of the pore-field area. (D) Pores. From Mullins et al. (2002), with permission from The Journal of Experimental Biology. Images courtesy of Donald and June Mullins.

Fig. 7.4 Scanning electron microscopy images of Blattella germanica oothecae, demonstrating the morphological basis of their permeability. (A) Proximal end of an ootheca showing the "escutcheon-shaped" vaginal imprint (arrow). (B) Magnification of the ventro-lateral escutcheon region; arrow indicates the pore field area. (C) Magnification of the pore-field area. (D) Pores. From Mullins et al. (2002), with permission from The Journal of Experimental Biology. Images courtesy of Donald and June Mullins.

found in the wrinkled region surrounding the "escutcheon-shaped" vaginal imprint on the proximal end (Mullins et al., 2002).

Because the barrier between mother and developing embryos is permeable, females that externally carry egg cases throughout gestation have the advantage of parceling water and other soluble materials to the embryos on an "as needed" basis. They also have some degree of behavioral control over the embryonic environment. Nymphs of B. germanica are known to settle in microhabitats where temperatures are favorable to their development (Ross and Mullins, 1995); it is probable that a female carrying an egg case acts similarly on behalf of her embryos. In most instances, hatch of the egg case is initiated while it is still attached to the mother. The activity level of the female increases significantly prior to hatch, indicating either that she can detect impending hatch, or that her increased activity level initiates it (D. E. Mullins and K. R. Tignor, pers. comm. to CAN).

Oviparity type B occurs in two subfamilies of Blattel-lidae. In the Blattellinae, at least nine species of Blattella and one species of the closely related Chorisia exhibit this reproductive mode (Roth, 1985). In the Pseudophyllo-dromiinae two species of Lophoblatta carry their oothe-cae externally throughout gestation. The first of these was found by LMR in the Amazon basin in 1967; a female Loph. brevis carrying an ootheca was collected on a banana plant, and the eggs hatched the following day. A second species with external egg retention, Loph. arlei, was taken from a bird nest. All other known Lophoblatta deposit their oothecae shortly after they are formed (Roth, 1968b).

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