Small, flying insects, particularly those<4 mm in length (Fig.10.1) such as
many Diptera and Coleoptera, ‘sail with the wind’ as aerial plankton during
long-distance movements and control their direction largely during takeoff and
landing (Johnson, 1969 ; Compton, 2002 ). Movement in such species clearly
combines aspects of both active and passive dispersal, and the factors influenc-
ing long-range movement may have more in common with passively-dispersing
non-insects (see below) than with larger flying animals. Larger insects associated
with freshwaters have more control over their flight direction and speed, and
achieve long-distance dispersal largely through active means (Fig.10.1).
Although active dispersal could be considered comparatively low risk, in
that the dispersers have the ability to seek out appropriate habitats, there
are other costs associated with active dispersal, including trade-offs with repro-
duction (Tanaka & Suzuki, 1998 ; Zera & Brink, 2000 ). Hence, flight efficiency
may play an important role in dispersal ecology. The subject of how flight
efficiency scales with body size and related morphological parameters has
received considerable attention in the literature. In general terms, the muscles
of larger animals perform the same mass-specific work for a lower metabolic
cost and are thus more efficient than those of small animals (Ellington,1991;
Harrison & Roberts,2000; Wakeling & Ellington, 1997 ). Aquatic insects broadly
fit this model, although there are some subtleties that may determine the
differences in dispersal potential among groups. Ellington ( 1991 ) suggested
that flight performance in the closely related dragonflies and damselflies scaled
positively with massþ0.27and negatively with mass0.23, respectively, and
Marden (1994) also suggested a negative scaling of body mass and maximum
sustainable flight performance in damselflies. It seems that damselflies may be
less efficient fliers than dragonflies. Based on estimates of lift and power from
wings, Wakeling and Ellington ( 1997 ) also suggested that dragonflies and dam-
selflies were adapted to different styles of flight, dragonflies being better adap-
ted to fly rapidly in large spaces and damselflies for manoeuvring in small
spaces. Based on these data it might be predicted that dispersal ability should
be positively and negatively correlated with body size in dragonflies and dam-
selflies, respectively, and that larger-bodied insects would generally have
a greater dispersal potential than small. It is also clear, however, that other
morphological characters, such as wing size (Guitie ́rrez & Mene ́ndez,1997 ;
McLachlan,1985 ) and wing muscle mass and lever length scaling (e.g. Schilder &
Marden,2004 ), may also play a major role in dispersal dynamics of dragonflies
and damselflies, and indeed other insects. At the same time, it is highly
unlikely that flight efficiency alone explains dispersal ability in freshwater
insects; other factors including levels of interspecific competition, habitat quality
and persistence will also have an influence on successful dispersal (Travis &
Dytham,1999 ; Ferriereet al., 2000;Clobertet al., 2001 ;Riberaet al., 2003 ;Vogler&
Ribera,2003 ).
BODY SIZE, DISPERSAL AND RANGE SIZE IN AQUATIC INVERTEBRATES 189