The ability of a cockroach to walk on vertical and inverted horizontal surfaces (like ceilings) is predicated on specific features of the tarsi. The tarsus is comprised of five subsegments or tarsomeres. Each of the first four of these may bear on its ventral surface a single, colorless pad-like swelling called the euplanta, plantula, or tarsal pulvillus. At the apex of the fifth tarsal subsegment is a soft adhesive lobe called the arolium, which lies between two large articulated claws (Fig. 2.3). The surface of the arolium is sculptured and bears a number of different types of sen-sillae. Both arolia and euplantae deform elastically to assure maximum contact with a substrate and to conform to the microsculpture of its surface. Little cockroach footprints left behind on glass surfaces indicate that secretory material aids in forming a seal with the substrate. Generally, when a cockroach walks on a smooth or rough surface, some of the euplantae touch the substrate, but the arolia do not. The tarsal claws function only when the insect climbs rough surfaces, sometimes assisted by spines at the tip of the tibiae. The arolium is employed primarily when a cockroach climbs smooth vertical surfaces such as glass; the claws spread laterally and the aroliar pad presses down against the substrate (Roth and Willis, 1952b; Arnold, 1974; Brousse-Gaury, 1981; Beutel and Gorb, 2001). These structures can be quite effective; an individual of Blattella asahinai that landed on a car windshield was not dislodged until the vehicle reached a speed of 45 mph (= 72 kph) (Koehler and Patternson, 1987).
Cockroach species vary in the way they selectively employ their tarsal adhesive structures. Diploptera punctata, for example, stands and walks with the distal tarsomeres raised high above the others, and lowers them only when climbing, but in Blaberus the distal tarsomeres are always
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).
in contact with the substrate (Arnold, 1974). Within a species, there may be ontogenetic differences. Unlike adults, first instars of B. germanica are 50% faster on glass than they are on rough surfaces, probably because they use euplantae more than claws or spines during locomotion (Wille, 1920).Variation in employing adhesive structures is related to the need to balance substrate attachment with the need to avoid adhesion and consequent inability to move quickly on various surfaces. Both Blatta orientalis and Periplaneta australasiae walk readily on horizontal glass surfaces if they walk "on tiptoe" with the body held high off the substrate. If the euplantae of the mid and hind legs are allowed to touch the surface, they become attached so firmly that the cockroach can wrench itself free only by leaving the tarsi behind, clinging to the glass (Roth and Willis, 1952b).
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