|312 HETEROCHRONY AND PAEDOMORPHOSIS
dwindle, progenetic reproduction is abandoned and the regular sexual cycle is resumed, with larvae molting to normal adults. Although this normal cycle omits the nymphal stages, Rack (p. 164) asserts that the progenetic cycle is much shorter and that progenetic forms are more fecund than normal adults.
2. Cousin (1938) studied the entomophagous chalcid wasp Melittobia chalybii. The typical form takes 90 days to metamorphose into a normal, winged adult with a long life span. But a "second form" develops in 14 days and has a short imaginal life. It is progenetic, with small wings and hypertrophied genitals functional right at eclosion. Eggs deposited on a nonparasitized host develop into second-form progenetics. Eggs from the same source deposited on a host already infected with Melittobia develop into the typical form.
3. When uncrowded on abundant rice plants, the plant hopper Nilaparvata lugens develops into a brachypterous (paedomorphic) feeding form. If the plant wilts or becomes too crowded, a normal- winged imago develops. The brachypterous imago is flightless; the normal-winged form flies to find new resources. The brachypterous imago is progenetic rather than neotenic since it has a shorter larval life than the normal-winged form (Kisimoto, 1956).
Neotenic Solitary Locusts:
Many species of locusts exhibit a continuous polymorphism reflecting their alternating life styles as solitary grasshoppers and migratory swarmers. The end forms, with obvious reference to their habits, have been named the solitaria and gregaria phases (Key, 1950; Kennedy, 1956; Uvarov, 1961; Dempster, 1963, for reviews). Of the transition to phase gregaria, Dempster writes: "Phase changes are complex and affect many characters such as size, shape, color, rate of development, behavior and fecundity; but basically they reflect a change in the locust's metabolism to favor greater mobility" (1963, p. 513). Gregaria locusts migrate in daylight hours as nymphs and adults; in comparison with solitaria locusts (Kennedy, 1956; Dempster, 1963), they are smaller and longer winged, have higher metabolic rates, eat more but retain less for growth and storage, and contain less water but more fat (to fuel their flight).
Kennedy (1956) argued, with much support from previous workers, that solitaria locusts should be viewed as a juvenilized phase, gregaria locusts as accentuated adults. He based his arguments on a large suite of characters, not just the larger wings of the gregaria phase; his conclusions have been generally accepted.