Wing Reduction and Flightlessness

All taxonomic groups of cockroaches include species with variably reduced or absent tegmina and hindwings, ex posing all or part of the dorsal surface of the abdomen. The exceptions are those groups in which the distal portion of the hindwing is set off by a transverse fold (e.g., Diplopterinae, Ectobiinae, Anaplectinae—Rehn, 1951). Wing reduction typically affects the hindwings more than the tegmina (Peck and Roth, 1992). Even when they are reduced, wings are always flexibly joined to the thorax. Adults with reduced wings can be distinguished from older nymphs, then, because the wing pads of the latter are nonflexible extensions of the posterior margins of the wing-bearing thoracic segments (Fisk and Wolda, 1979). Although in some cockroach groups apterous species are tiny and may be passed over by collectors because they resemble nymphs (Mackerras, 1968a), some of the largest known cockroaches (Macropanesthia) also lack wings.

Based on information in Rehn (1932b) and Roth and Willis (1960), Roff (1990, Table 8) estimated that more than 50% of all cockroaches and 50-60% of temperate species lack the ability to fly. Vastly different figures also have been published. Roff (1994) indicated that just 4% of cockroaches are flightless in both sexes, and 24% are sexually dimorphic, with males flying and females flightless (data from North America, French Guiana, Africa, and Malagasy). There are reasons to be cautious when assessing cockroach flight ability. First, only a fraction of the more than 4000 known cockroach species are included in these estimates; volant canopy species in particular may be underestimated. Second, flight capability in cockroaches is typically based on published descriptions of wing morphology in museum specimens. The possession of fully developed wings, however, does not necessarily mean that a cockroach can fly (Farnsworth, 1972; Peck and Roth, 1992).

A more accurate measure of cockroach flight capability may lie in the color of the thoracic musculature of freshly killed insects. Kramer (1956) found that the pterothoracic musculature of apterous, brachypterous, and flightless or feebly flying macropterous cockroaches appears hyaline white, while that of strong fliers is opaque and conspicuously pink (Table 2.1). These color differences are correlated with distinct metabolic differences, as reflected in enzymatic activity and oxygen uptake (Kramer, 1956). Consequently, cockroaches with white musculature may not be able to release energy rapidly enough to sustain wing beating (Farnsworth, 1972). In cockroaches with pink musculature, the muscles of the mesothorax and metathorax are equally pigmented. One exception is the "beetle" cockroach D. punctata (= dytis-coides), which derives its common name from the fact that the somewhat reduced, hardened tegmina resemble elytra and cover a pair of long hindwings (Fig. 2.9). In this species the mesothoracic muscles are hyaline white, but the metathorax bearing the elongated hindwings con-

Table 2.1. Wing development and its relationship to pigmentation of the thoracic musculature. Based on Kramer (1956) and Roth and Willis (1960).

Color of pterothoracic musculature

Table 2.1. Wing development and its relationship to pigmentation of the thoracic musculature. Based on Kramer (1956) and Roth and Willis (1960).

Color of pterothoracic musculature

Mesothorax

Metathorax

Cockroach species

(wing condition)1

(wing condition)

Blaberus craniifer

Pink (M)

Pink (M)

Blaberus giganteus

Pink (M)

Pink (M)

Blatta orientalis

White (R)

White (R)

Blattella germanica

White (M)

White (M)

Blattella vaga

Pink (M)

Pink (M)

Cryptocercus punctulatus

White (A)

White (A)

Diploptera punctata

White (R)

Pink (M)

Eurycotis floridana

White (R)

White (R)

Nauphoeta cinerea

White (R)

White (R)

Neostylopyga rhombifolia

White (R)

White (R)

Parcoblatta pennsylvanica

Male

Pink (M)

Pink (M)

Female

White (R)

White (R)

Parcoblatta virginica

Male

Pink (M)

Pink (M)

Female

White (R)

White (R)

Periplaneta fuliginosa

Male

Pink (M)

Pink (M)

Female

White (M)

White (M)

Periplaneta brunnea

Male

Pink (M)

Pink (M)

Female

White (M)

White (M)

Periplaneta australasiae

Male

Pink (M)

Pink (M)

Female

White (M)

White (M)

Pycnoscelus surinamensis2

Pink (M)

Pink (M)

Rhyparobia maderae

Pink (M)

Pink (M)

Supella longipalpa

Male

Pink (M)

Pink (M)

Female

White (R)

White (R)

1M = macropterous, R = reduced, A = absent. 2Female morphs with reduced wings exist.

1M = macropterous, R = reduced, A = absent. 2Female morphs with reduced wings exist.

tains pigmented muscle (Kramer, 1956). Macropterous adults with white musculature include Blattella germanica, females of Supella longipalpa (= supellectilium), and three species of Periplaneta. Both sexes of B. germanica and Blattella vaga have fully developed wings (see Plate 5 of Roth and Willis, 1960), but B. germanica is incapable of sustained flight (Brenner et al., 1988).2 The rosy flight muscles of B. vaga are an indication that it is volant, but its flight behavior is unknown. The Asian cockroach Blattella asahinai is morphologically very similar (Lawless,

2. It is, however, a frequent flier on airplanes (Roth and Willis, 1960).

1999) and very closely related (Pachamuthu et al., 2000) to B. germanica, but flies readily and strongly (Brenner et al., 1988); presumably, dissections would indicate that it has pigmented flight muscles. Males of Su. longipalpa are fleet runners and can take to the air for short distances, but females are unable to fly (Hafez and Afifi, 1956). Another example of a macropterous but flightless species is Thorax porcellana (Epilamprinae). Both sexes are fully winged, but only the male uses them for short flights and only rarely (Reuben, 1988).

The correlation between flight muscle pigmentation and the physiological ability to sustain flight has been examined most extensively in P. americana. In tests on laboratory strains tethered females (white flight muscles) could sustain no more than a 3-12 sec flight, compared to 5-15 min in males (pink flight muscles). Moreover, freshly ecdysed male P. americana have white pterotho-racic muscles and flight behavior similar to that of adult females: they flutter weakly or plummet when tossed into the air. The flight behavior of these young males changes in conjunction with the postmetamorphic development of pink pigmentation in their musculature (Kramer, 1956; Farnsworth, 1972; Stokes et al., 1994). In the tropics P. americana is reportedly an excellent flyer, and is known in some locales as the "Bombay canary." It has been observed flying out of sewers and into buildings. It was also spotted in a German zoo flying distances of up to 30 m, in fairly straight lines or in flat arcs about 0.5 to 1.5 m above the ground (Roth and Willis, 1957). It is unclear, however, whether these volant P. americana are males only, or if both sexes in natural populations can fly. Rehn (1945) indicated that the flying ability of Periplaneta (species unspecified) is "often exercised and by both sexes." Female P. americana from laboratory cultures in two U.S. locations and one in Germany, however, remained earthbound during flight tests (Kramer, 1956). Appel and Smith (2002) report that P. fuliginosa females with fully formed oothecae are capable of sustained flight on warm, humid evenings in the southern United States, but laboratory-reared females of this species sank like rocks when tossed in the air (Kramer, 1956). Perhaps females lose the ability to fly when raised in culture. At least one study demonstrated that flight initiation in P. americana was significantly affected by the temperature at which they were reared (Diekman and Ritzman, 1987), and flight performance in other insects is known to quickly suffer under laboratory selection (Johnson, 1976).

A physiological change in flight musculature no doubt precedes or accompanies morphological wing reduction, but may be the only modification if the tegmina and wings have a functional significance other than flight. Full-sized wings may be retained in flightless species be cause they may act as parachutes, controlling the speed and direction of jumps and falls. German cockroaches, for example, will glide short distances when disturbed (Koehler and Patternson, 1987). Tegmina and wings may be used as tools in territorial or sexual signaling; males in several species flutter their wings during courtship. They also may serve as stabilizers during high-speed running, as physical protection for the abdomen and associated tergal glands, in visual defense from enemies (crypsis, mimicry, aposematicism), and, in rare cases, as shelter for first instars.

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