Islands

Darwin (1859) first suggested that the isolation imposed by living on an island selects for flightless morphologies, because sedentary organisms are less likely to perish by being gusted out to sea. More recent authors, however, have questioned the hypothesis (e.g., Darlington, 1943). For one thing, scale is not taken into account. Conditions are different for a large insect on a small island versus a tiny insect on a substantial one (Dingle, 1996). Roff (1990) analyzed the wing condition of insects on oceanic islands versus mainland areas (corrected for latitude) and found no correlation between island life and a sedentary lifestyle. Denno et al.'s (2001a) work on planthoppers in the British Virgin Islands also supports this view.

The observation that a flightless cockroach lives on an island does not necessarily mean that the wingless condition evolved there. Cockroaches have greater over-water dispersal powers than is generally assumed, because they raft on or in floating debris and vegetation (Peck, 1990; Peck and Roth, 1992). Moreover, cockroaches that live under bark or burrow in wood or other dead vegetation may be the most likely sailors; this category includes a relatively high percentage of wing-reduced species (discussed below). Trewick (2000) recently analyzed DNA sequences in the blattid Celatoblatta, a flightless genus found in New Zealand and in the Chatham Islands, habitats separated by about 800 km of Pacific Ocean. The island populations were monophyletic, and probably dispersed from New Zealand to the islands by rafting sometime during the Pliocene (2-6 mya). Members of this genus are known to shelter in logs during the day.

When six small mangrove isles off the coast of Florida were experimentally sterilized, Latiblattella rehni and an undescribed species in the same genus were early re-invaders on several of them (Simberloff and Wilson, 1969). Males of Lat. rehni have fully developed, "very delicate" (Blatchley, 1920) wings; those of the female are slightly reduced, but it is unknown if they are functional. Colonization, then, could have been by active or passive flight, or by rafting. The Krakatau Islands offered a unique opportunity to study the reintroduction of cockroaches into a tropical ecosystem from a sterile baseline after a series of volcanic eruptions in 1883 stripped them of plant and animal life. A 1908 survey found a few cockroach species already present, with a subsequent steep colonization curve that flattened out after the 1930s (Thornton et al., 1990). The 17 species reported from the islands by 1990 include pantropical species (P. americana, Blatta orientalis) probably introduced by humans, fully winged species (e.g., Balta notulata, Haanina major), those with reduced wings (Lobopterella dimidiatipes), and species in which there is a great deal of variation in wing reduction in both sexes (e.g., Hebardina concinna). Neostylopyga picea, which has short tegminal pads and lacks wings, also is present on the islands and probably arrived by rafting. It is generally found in humus and decaying wood (Roth, 1990a).

Studies in the Galapagos offer the best evidence that the evolution of flightlessness may occur on islands. Eighteen species are reported on the Galapagos (Peck and Roth, 1992). Of these, the introduced or native (naturally occurring tropical American and Galapagos) cockroaches are fully winged as adults, except for female Sym-ploce pallens. The five endemic species are all partially or wholly flightless. Peck and Roth (1992) suggest that three natural colonization events took place. First, an early colonization by Ischnoptera and loss of flight wings in three descendent species, a later colonization by Chorisoneura and partial reduction of flight wings in two descendent species, and lastly, a recent colonization by Holocampsa nitidula and perhaps another Holocampsa sp. These authors give a detailed analysis of the process of wing reduction in the studied cockroaches, and conclude that their data fit the generalization that loss of flight capability often accompanies speciation on islands. The authors do note, however, that the flightless condition "may not be a result of island life per se, but may be a specialization for life in more homogenous leaf litter or cave habitats at higher elevations on the islands."

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