The adult beetle flies nocturnally throughout the warmer and drier part of the year, between late spring and early autumn. Its phototropism was first noted by Hudson (1892) thus: “It is greatly attracted by light and this propensity frequently draws it on summer evenings to invade ladies' drawing rooms when its sudden and noisy arrival is apt to cause needless consternation among the inmates.” Neither male nor female feed: their adult life span in the field is about two weeks. The midgut which in late larvae is a large tube reaching a length of 8–10 cm, is reduced in the adult to a vestige of degenerate tissue linking fore- and mid-guts (Fig. 1). The fore-gut undergoes the converse transformation: the short tube of the larva extends to become a large air-filled sac extending into the abdomen, dilating as the initially massive fat body regresses, and in the female as eggs are laid. The hind gut remains relatively unchanged through metamorphosis but largely loses its heavy muscular layer in the adult. The cryptonephric malpighian tubules remain functional in the adult, retaining their role in water conservation. Prionoplus will take water in captivity but it is not known to what extent the beetle drinks in nature.
Gametogenesis is completed early in the pupal phase, and maturation of spermatozoa and ova is complete before eclosion. The gonads are indeed undergoing resorption throughout adult life (unpublished observations). The adult phase then is solely concerned with fertilization, distribution, host location and oviposition.
The sex ratio of beetles that emerged from enclosed logs (154 ♂ ♂ : 190 ♀ ♀) does not depart significantly from an expected 1 : 1 ratio.