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

Catherine A Tauber,Maurice J Tauber,Jon G Giffin

doi:10.14411/eje.2007.100

Abstract

Five flightless species of Micromus are known from the Hawaiian Archipelago; only one, the rare Micromus 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 and 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.

Highlights

Secondary loss or reduction of wings – and the resulting loss of flight – has occurred numerous times among pterygote insects (e.g., Roff, 1990; Wagner & Liebherr, 1992)
Collection records to date [our own, those from specimens in the Bishop Museum and the Cornell University Insect Collection, and in the literature (Zimmerman, 1957)] indicate that both M. usingeri and the fully macropterous M. longispinosus have relatively broad distributions on the Island of Hawai‘i, but that each is restricted altitudinally
Intermediate specimens were collected in the districts of Ka‘u, North Hilo, and North Kona in a variety of habitats, and generally at elevations between those of M. usingeri and the macropterous M. longispinosus (1,700–2,400 m) (Table 1)

Summary

Introduction

Secondary loss or reduction of wings – and the resulting loss of flight – has occurred numerous times among pterygote insects (e.g., Roff, 1990; Wagner & Liebherr, 1992). Eleven species within five hemerobiid lineages show hind-wing reduction or loss; this pattern indicates that brachyptery and flightlessness have evolved repeatedly in the group (Oswald, 1996). The incidences of wing reduction and loss of flight tend to increase at high altitudes and on isolated landmasses surrounded by inhospitable terrain or water (Roff, 1990; Gillespie & Roderick, 2002). Of the ~23 known species in the presumed monophyletic radiation of endemic Hawaiian hemerobiids (subfamily Microminae, genus Micromus), five have undergone striking modifications of the forewings, drastic hind-wing reduction, and loss of the ability to fly (Perkins, 1899; Zimmerman, 1957). It is reasonable to conclude that flightlessness evolved independently in at least three, and perhaps all five, of these species (e.g., see Zimmerman, 1957; Oswald, 1996)

Methods

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Conclusion