Pronotum

The large, shield-shaped pronotum is a defining characteristic of cockroaches and its size, shape, curvature, and protuberances have systematic value in certain groups (e.g., Perisphaeriinae, Panesthiinae). Some cockroaches are more strongly hooded than others, that is, the head ranges from completely covered by the pronotum to almost entirely exposed. In some species the pronotum is flat, in others it has varying degrees of declivity. At its extreme it may form a cowl, shaped like an upside down U in section. The border of the pronotum may be recurved to varying degrees, forming a gutter around the sides, which sometimes continues into the cephalic margin. The majority of species of Colapteroblatta, for example, have the lateral wings of the pronotum deflexed and the edges may be ridged or swollen (Hebard, 1920 [1919]; Roth, 1998a, Fig. 1-6). In a few cases the pronotum can resemble the headpiece of certain orders of nuns (Fig. 1.1A). Some species of Cyrtotria have pronota perforated with large, semilunar pores in both sexes; these may be the openings of glands (Fig. 1.1B) (Shelford, 1908). The shape of the pronotum can vary within a species, with distinct forms correlated with varying degrees of wing re-

Fig. 1.1 Variations in pronotal morphology. (A) Female of Cyrtotria marshalli, three-quarter view. (B) Female of Cyrtotria pallicornis, three-quarter view; note large lateral pores. (C) Male of Princisia vanwaerebeki, lateral view. (D) Female of Pilema mombasae, dorsal view. (E) ditto, lateral view. After Shelford (1908) and Van Herrewege (1973). Not drawn to scale.

Fig. 1.1 Variations in pronotal morphology. (A) Female of Cyrtotria marshalli, three-quarter view. (B) Female of Cyrtotria pallicornis, three-quarter view; note large lateral pores. (C) Male of Princisia vanwaerebeki, lateral view. (D) Female of Pilema mombasae, dorsal view. (E) ditto, lateral view. After Shelford (1908) and Van Herrewege (1973). Not drawn to scale.

Fig. 1.2 Male Microdina forceps (Panesthiinae) from India. Photo by L.M. Roth.

duction (e.g., African Ectobius—Rehn, 1931). Both males and females of Microdina forceps have the anterior pronotal margins extended into a pair of curved spines, resembling the forceps of earwigs or the mandibles of staghorn beetles (Fig. 1.2) (Roth, 1979b). In females these are about 2 mm long, and in males they are slightly longer (2.5 mm). In Bantua valida the lateral margins of the prono-tum in both sexes are curved upward, but only in the fe male are the caudad corners prolonged into "horns" (Kumar, 1975).

Functionally, the pronotum is a versatile tool that can serve as a shield, shovel, plug, wedge, crowbar, and battering ram. Those cockroaches described as "strongly hooded,"with the head concealed under the extended anterior edge of the pronotum, are often burrowers. The large, flat pronotum of Blaberus craniifer, for example, serves as a wedge and protects the head when used in the oscillating digging motion described by Simpson et al. (1986). In museum specimens of Pilema spp. the channel between the pronotal disc and lateral bands is often chocked with dirt, leading Shelford (1908) to conclude that the pronotum (Fig. 1.1D,E) is used in digging the neat round holes in which these cockroaches are found. Adult Cryptocercus have been observed using the pronotum as a tool in two different contexts. When they are cleaning and maintaining their galleries, the insects use the pronotum as a shovel to move frass and feces from place to place and to tamp these materials against gallery walls (CAN, unpubl. obs.). During aggressive encounters the pronotum is used to block access to galleries and to push and butt intruders (Seelinger and Seelinger, 1983; Park and Choe, 2003b). In male Nauphoeta cinerea, combatants try to flip rivals onto their backs by engaging the edge of their pronotum under that of their opponents (Ewing, 1967). In species with strong sexual differences in pronotal morphology, dimorphism is likely related to sexual competition among males. In Elliptorhina, Princisia, and Gromphadorhina, males have heavy, well-developed knobs on their pronota and use them to battle rivals (Fig. 1.1C) (Van Herrewege, 1973;Beccaloni, 1989). When males charge, their knobbed pronotal shields come together with an audible sound (Barth, 1968c). In Geo-scapheini (Blaberidae), males often have conspicuous pronotal tubercles that are absent in the female, and have the anterior edge thickened and prominently upturned (Walker et al., 1994); Macropanesthia rhinoceros is named for the blunt, horn-like processes projecting from the surface of the pronotum in males (Froggatt, 1906). Individuals of M. rhinoceros are most often observed above ground when they have "fallen on to their backs and are unable to right themselves" (Day, 1950). It is unknown if these are all males, and the result of nocturnal battles. The allometry of male combat weaponry has not been examined in cockroaches.

In some cockroach species the pronotum is used to both send and receive messages and thus serves as a tool in communication. In N. cinerea there are about 40 parallel striae on the ventral surface of the latero-posterior edges of the pronotum. The insects stridulate by rubbing these against the costal veins of the tegmina (Roth and Hartman, 1967). The pronotum is also very sensitive to tactile stimulation in this species. Patrolling dominant males of N. cinerea tap members of their social group on the pronotum with their antennae, evoking a submissive posture in lower-ranking members (Ewing, 1972). Similarly, reflex immobilization in Blab. craniifer can result from antennal tapping of the pronotal shield by another individual (Gautier, 1967).

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