Vertical Stratification

In lowland Costa Rican rainforest individuals space themselves in the vertical dimension during their active period (Schal and Bell, 1986).There is intersexual and ontogenetic variation in the behavior, with males tending to perch higher in the vegetation than females (Fig. 3.4). This is not simply a function of perch availability, since many potential perch sites remain unoccupied. Perch height was generally associated with flight ability. Adult females of E. involucris, Nesomylacris sp., and Hyporich-noda reflexa are either wingless or have very short wings, and they perch close to the ground. Epilampra unistilata, Xestoblatta hamata, and X. cantralli comprise an intermediate group; all are good fliers and after spending the day in ground litter, fly to higher perches. The arboreal pseudophyllodromiine species (Imblattella n. sp. G, and Cariblatta imitans) are excellent fliers and perch higher than the intermediate group at night. Except for Imblattella spp., early instars are located in ground litter where partition occurs; as nymphs develop they gradually perch higher in the foliage (Schal and Bell, 1986).

Vertical stratification during the active period has been observed in subtropical and temperate cockroaches as well. In the forests and grasslands of eastern Kansas, six species (Parcoblatta spp., Ischnoptera spp.) are distributed vertically at night among grasses, shrubs, and trees (Gor

Fig. 3.4 Vertical distribution of male and female cockroaches during the night in the Costa Rican rainforest. Males, open box; females, black box. From Schal and Bell (1986).

ton, 1980). Males are good fliers and are generally located higher than females, most of which remain on or near the ground. Females seldom fly and, except for Parc. pennsyl-vanica, all have reduced wings. The inability to fly, however, is not always correlated with low perch height. Both nymphs and brachypterous females of Ectobius sylvestris walk on trunks into tree canopies (Vidlicka, 1993).

Cockroaches appear to sort themselves in the vertical dimension via their differential sensitivity to zones of temperature, humidity, and wind currents (Edney et al., 1978; Appel et al., 1983). Schal (1982) found significant differences in these variables up to a height of 2 m in the tropical forest subcanopy. In one experiment, individually marked E. involucris were blinded, then placed at heights where they usually do not occur; all individuals migrated back to their typical perch zone. This stratification along micrometeorological gradients relates to the ascent of warm air and pheromone dispersion at night. Females emit sex pheromones while perching, and temperature inversions carry the pheromones aloft. Males perching higher than females would be able to detect rising pher-omones and locate receptive females. Perching behavior in adults, then, is primarily a mate-finding strategy (Schal, 1982; Schal and Bell, 1986), a conclusion supported by the observations of Gorton (1980). Among the temperate species he studied in Kansas, males were generally found high, females low, and copulating pairs in between. Vertical stratification may also be related to communication between males and females in desert cockroaches (Hawke and Farley, 1973), but data are lacking to support this idea or to exclude other explanations.

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