Reduction and Loss of the Egg Case

In most oviparous type A cockroaches, the ootheca is a hard, dark, stiff structure completely covering the eggs. The dorsal keel is structurally complex, and the outer covering contains calcium oxalate crystals. These crystals comprise 8-15% of the dry weight of the ootheca in P

americana, and are thought to have a structural and protective function (Stay et al., 1960; Rajulu and Ren-ganathan, 1966), just as they do in plants that possess them (Hudgins et al., 2003). The oothecal casing is thinner and less rigid in species that externally carry the egg case (oviparous type B); calcium oxalate crystals are sparse in both B. germanica and Loph. brevis (Roth, 1968b). Ovoviviparous type A cockroaches typically produce a thin, soft, lightly colored ootheca that lacks a keel and which in some species only partially covers the eggs, particularly in later stages of gestation (Roth, 1968a) (Fig 7.5A); calcium oxalate is absent. This type of egg case is produced by Blaberidae and also Sliferia, one of few Blattellidae that retract their ootheca into a brood sac (Stay et al., 1960; Roth, 1968a). The nature of the ootheca, then, changes in parallel with stages of internalization of the egg case. It goes from having a rigid outer casing in those species that abandon the egg case, to a flexible, soft membrane in those that have internalized it. It has intermediate properties in those cockroaches that carry the ootheca externally during gestation, and has been completely lost in one derived lineage (Geoscapheini: ovoviviparous type B) (Roth and Willis, 1958a; Roth, 1968a, 1970a). Females exhibit a parallel regression of the morphological structures associated with oothecal production (reviewed by Nalepa and Lenz, 2000).

Oviparous cockroaches in protected environments, like social insect nests, also may exhibit reduction or loss of the egg case. The ootheca of Attaphila fungicola, for example, lacks a keel (Roth, 1971a), and several species of Nocticolidae have thin, transparent oothecal cases. Nocti-cola termitophila apparently lays its eggs singly, without any external covering (Roth, 1988). Termites, the "social cockroaches" (Chapter 9), exhibit a parallel loss of protective egg cases. The basal termite Mastotermes dar-winiensis packages its eggs within a thin, flexible outer covering that lacks keel. The site and mode of production, associated morphological structures in the female, parallel arrangement of eggs, and discrete, tanned outer covering together indicate that the ootheca of Mastotermes is homologous with those of cockroaches (Nalepa and Lenz, 2000). All other termites lay their eggs singly, without a covering. Both the heart of a social insect colony and the brood sacs of live bearing cockroaches are moist, protected sites for incubating eggs, allowing for the reduction and eventual elimination of defensive structures in evolutionary time. The oothecal case is 86-95% protein (Table 4.5), so "it is no wild supposition that in the course of time the chitinous ootheca, being in these species a work of supererogation, will disappear" (Shelford, 1912b). Perhaps the main reason that the ootheca has not been completely eliminated in most ovoviviparous cockroaches is because it determines the orderly arrangement of eggs and therefore assures contact and exchange of water and other materials between each egg and the wall of the brood sac (Rugg and Rose, 1984b). A study of the Geo-scapheini whose eggs are incubated in a disordered mass in the brood sac (Rugg and Rose, 1984c) (Fig. 7.5B) is the logical focal group for testing this hypothesis.

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