Flightless Hawaiian Hemerobiidae (Neuroptera): Comparative morphology and biology of a brachypterous species, its macropterous relative and intermediate forms

Abstract

Five flightless species of Micromus are known from the Hawaiian Archipelago; only one, the rare Mcromus usingeri, is reported from the Island of Hawai'i. Herein, we report the natural occurrence of intermediates-between this brachypterous species and its near relative, the macropterous Micromus longispinosus. We compare some morphological aid life-history characteristics of the two species and the intermediates. Our study shows that: (1) The two closely related species are broadly distributed on Hawai'i, but they appear to be allopatric altitudinally. (2) M. usingeri is associated with a cool, misty, high-altitude environment, M. longispinosus with warmer, rainy conditions at lower elevations. The intermediates occur in both types of situations and generally at intermediate elevations. (3) The macropterous M. longispinosus has large, oblong, flexible, membranous forewings and hind wings. In contrast, the brachypterous M. usingeri has convex, shortened, elytra-like forewings with reticulate venation, and very small, thick, triangular, stub-like hind wings with greatly reduced venation. The wings of intermediate specimens exhibit a broad range of variation between the two species. (4) Several characteristics of wing venation are highly correlated with reduced wing size; others are not. (5) Aside from the wings, adults of M. usingeri and M. longispinosus differ in relatively few morphological features, most notably the antennal and metatibial length, prothoracic length, mesothoracic length and width, and the length of the spine-covered process on the posteroventral margin of the male T9+ectoproct. The intermediate specimens are variable in adult characteristics, but they generally fall between the two species. (6) Egg size and larval characteristics (except the body length of the fully-fed first and third instars) do not differ between the two species. (7) The evolution of the wing variation is discussed.