Tarsal Morphology Relation to Environment

Cockroaches vary in their ability to climb (i.e., escape) glass containers (Willis et al., 1958). This is due principally to the development of the arolium, which varies in size, form, and sculpturing and may be absent in some species (Arnold, 1974). Blatta orientalis, for example, has subobsolete, nonfunctional arolia and is incapable of climbing glass (Fig. 2.3). Euplantae may also differ in size and shape on the different tarsomeres, be absent from one or more, or be completely lacking. The presence or absence of these adhesive structures can be used as diagnostic characters in some genera (e.g., the genus Allacta has euplantae only on the fourth tarsomere of all legs), but are of minor taxonomic significance in others (e.g., the genera Tivia, Tryonicus, Neostylopyga, Paratemnop-teryx) (Roth, 1988,1990b, 1991d). Intraspecifically, variation may occur among populations, between the sexes, and among developmental stages (Roth and Willis, 1952b; Mackerras, 1968a). In Paratemnopteryx (= Shawella) couloniana and Neotemnopteryx (= Gislenia) aus-tralica euplantae are acquired at the last ecdysis (Roth, 1990b).

Although arolia and euplantae are considered adaptive characters related to functional requirements for climbing in different environments (Arnold, 1974), it is not currently obvious what habitat-related features influence their loss or retention in cockroaches. Adhesive structures are frequently reduced or lost in cave cockroaches, perhaps because clinging mud or the surface tension of water on moist walls reduces their effectiveness (Mackerras, 1967c; Roth, 1988,1990b, 1991a). It would be instructive to determine if the variation in adhesive structures exhibited by different cave populations of species like Paratemnopteryx stonei can be correlated with variation among surfaces in inhabited caves. Arolia are absent in all Panesthiinae (Mackerras, 1970), and the two cockroaches listed by Arnold (1974) as having both arolia and euplantae absent or "only vaguely evident"—Arenivaga investi-gata and Cryptocercus punctulatus—are both burrowers. Nonetheless, the loss of arolia and euplantae is not restricted to cave and burrow habitats (Roth, 1988); many epigean species lack them. Arnold (1974) found it "surprising" that the tarsal features are so varied within cockroach families and among species that inhabit similar environments. A number of authors, however, have emphasized that it is the behavior of the animal within its habitat, rather than the habitat itself, that most influences locomotor adaptations (Manton, 1977; Evans and For-sythe, 1984; Evans, 1990). The presence and nature of appendage attachment devices is thought to be strongly associated with a necessity for negotiating smooth, often vertical plant surfaces (Gorb, 2001). Thus in a tropical forest, a cockroach that perches or forages on leaves during its active period may retain arolia and euplantae, but these structures may be reduced or lost in a species that never ventures from the leaf litter. Pulvilli and arolia are very well developed, for example, in Nyctibora acaciana, a species that oviposits on ant-acacias (Deans and Roth,

Fig. 2.3 Adhesive structures on the legs of cockroaches. Top, euplantae (arrows) on tarsal segments of two cockroach species. (A) Hind tarsus of male Opisthoplatia orientalis; (B) hind tarsus of male Comptolampra liturata. From Anisyutkin (1999), with permission of L.N. Anisyutkin. Bottom, apical and dorsal view of the pretarsi of the prothoracic legs in two cockroach species, showing the claws and arolia. Left, a cockroach able to walk up a vertical glass surface (male Periplaneta americana); right, one unable to do so (female Blatta orientalis). a = arolium; b = aroliar pad; c = tarsal claw. After Roth and Willis (1952b).

Fig. 2.4 Oxygen consumption while running on a treadmill: a cockroach built for speed (Peri-planeta americana) versus one built for power (Gromphadorhina portentosa). Oxygen peaks rapidly in P. americana, and afterward the insect recovers rapidly. There is a lag time before oxygen peaks in G. portentosa, and a slow recovery time while the insect "catches its breath." Note difference in scale ofy-axis. Reprinted from Herreid and Full (1984), with permission from Elsevier.

Fig. 2.4 Oxygen consumption while running on a treadmill: a cockroach built for speed (Peri-planeta americana) versus one built for power (Gromphadorhina portentosa). Oxygen peaks rapidly in P. americana, and afterward the insect recovers rapidly. There is a lag time before oxygen peaks in G. portentosa, and a slow recovery time while the insect "catches its breath." Note difference in scale ofy-axis. Reprinted from Herreid and Full (1984), with permission from Elsevier.

2003). In cockroaches that possess them, variation in sculpturing on the arolia may function in maximizing tenacity and agility on specific plant surface morphotypes (Bernays, 1991). Many species of tropical cockroach do not run when on leaves, but instead stilt-walk (WJB, pers. obs.). The slow leg movements produce little vibration in the substrate, and may allow them to ease past spiders without eliciting an attack, a phenomenon called "vibro-crypticity" (Barth et al., 1988).

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