Flight muscle breakdown in the green-veined white butterfly, Pieris napi (Lepidoptera: Pieridae)

Fredrik Stjernholm,Bengt Karlsson

doi:10.14411/eje.2008.012

Abstract

Flight is important for insects but also incurs costs in terms of reduced reproductive reserves. Recent studies on butterflies have shown that thorax mass and nitrogen content decrease over the adult lifespan, suggesting that flight muscle breakdown may also occur in butterflies. However, unlike other insects known to resorb flight muscles, butterflies will continue to fly throughout the reproductive period. Nonetheless, use of nutrients from flight muscles for reproduction has the potential to improve the reproductive output considerably. In this study we have tested to what extent female Pieris napi L. (Pieridae) butterflies actually do breakdown flight muscles. By comparing muscle mass in recently emerged and older free-flying females we show that mass and nitrogen content of the two most important groups of flight muscles each decrease by more than 50% over the adult lifespan. The significance of this finding is discussed in relation to reproduction and flight in butterflies.

Highlights

Flight muscle breakdown has been hypothesised to occur in butterflies, since thorax mass and nitrogen content decrease with age in many species (Karlsson, 1994, 1998; Stjernholm & Karlsson, 2000; Norberg & Leimar, 2002; Stjernholm et al, 2005)
The comparison of flight muscle size in recently emerged and older free-flying P. napi females demonstrated that the mass and nitrogen content of the two major groups of flight muscles in long-lived females is less than 42% of the levels in recently emerged females, confirming that much of the musculature is broken down over the lifespan
Flight muscle breakdown has previously been recorded in insects of several orders (Johnson, 1976; Sandlan, 1979; Zera & Denno, 1997; Marden, 2000), but has up till only been found in association with a lifestyle where flight primarily is important before but not during the reproductive period

Summary

Introduction

It increases the range over which food, partners and suitable habitat for breeding can be found, and in some species flight is a prerequisite for long distance migration. Flight is used by many species for evading predators, courtship and male-male contests. In some insects the potential benefits of flight are counterbalanced by the cost of the flight apparatus, and flight ability has been lost (Roff, 1986, 1990; Sattler, 1991; Zera & Denno, 1997). In situations where resources are scarce and/or flight is of marginal importance fitness may be higher if resources are used directly for reproduction than if they are used to build, maintain and fuel the flight apparatus (Roff, 1986; Zera & Denno, 1997). Sandlan (1979) presented circumstantial evidence for flight muscle degeneration in starved adults of the parasitoid wasp Coccygomimus turionellae; flight capability decreased but egg production could continue

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