Journal of Insect Physiology 47 2001 83–94 www.elsevier.comlocatejinsphys
Effects of suppressed oviposition activity and flight muscle histolysis on food consumption and ovarian development in a
wing-dimorphic cricket: an explanation for sporadic conclusions related to physiological trade-offs
Seiji Tanaka
Department of Insect Physiology and Behavior, National Institute of Sericultural and Entomological Science, Owashi 1-2, Tsukuba, Ibaraki 305- 8634, Japan
Received 10 March 2000; accepted 19 May 2000
Abstract
The effects of deprivation of oviposition substrate on food consumption and egg production were compared between the long- winged LW and the short-winged SW morph of a cricket, Modicogryllus confirmatus, to determine how suppressed oviposition
activity would influence these traits in each wing morph. Food consumption was greatly suppressed in females deprived of ovi- position substrate 2OS compared to those given access to it
+ OS during the 2-week feeding trial in the SW morph but not in
the LW morph. Some LW females shed their hindwings and histolyzed the flight muscles. Such de-alated LW DLW morphs tended to consume more food than intact LW ILW morphs. In all morphs, ovarian weight was heavier under 2OS conditions
than under +
OS conditions during the second week of adulthood, although the differences were greater in SW morphs than in ILW morphs. In DLW morphs in which flight muscle histolysis was induced by artificial de-alation at adult emergence, the temporal
changes in ovarian weight were similar to those of SW morphs. In SW morphs, food consumption was also significantly reduced when ovipositing females were deprived of oviposition substrate
for 2 days compared to those allowed to oviposit continuously, but food consumption was not reduced in ILW or DLW morphs. SW females from which one ovary was extirpated at adult emergence, SW 2o, also showed a significant difference in food
consumption when treated as above, indicating that food consumption was not determined simply by the number of ovarian eggs. The crop content was positively correlated to food consumption and smaller under 2OS conditions than under
+ OS conditions.
The 2-day deprivation of oviposition substrate caused no significant difference in the total number of deposited and ovarian eggs in any group, but the ovarian mass of developing oocytes tended to be smaller under 2OS than under
+ OS conditions, particularly
in SW morphs. These results indicate the possibility that some inconsistent results and conclusions discussed in recent studies, concerning the
physiological trade-offs between flight capability and reproduction, were caused by the suppressed oviposition activity and failure to recognize the occurrence of flight muscle growth and histolysis in the test crickets.
2000 Elsevier Science Ltd. All rights reserved.
Keywords: Wing dimorphism; Physiological trade-off; Oviposition; Food consumption; Egg production; Flight muscle histolysis; Crickets
1. Introduction
Wing dimorphism is commonly found in various insect orders including Orthoptera, Dermaptera, Zorap-
tera, Psocoptera, Thysanoptera, Hemiptera, Homoptera, Coleoptera, Lepidoptera, Diptera and Hymenoptera
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2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 1 9 1 0 0 0 0 0 0 9 4 - 9
Johnson, 1969; Borror et al., 1976; Harrison, 1980; Din- gle, 1985; Roff, 1986; Fujisaki, 1994. In general, two
morphologically distinct morphs, long-winged LW and short-winged SW, occur and exhibit various physio-
logical and behavioral differences. In some groups, apterous morphs are known, but in terms of the dispersal
capability, they are equivalent to the SW morph. The LW morph normally has well-developed flight muscles
and is regarded as a migratory morph. On the other hand, the SW morph with reduced wings and flight muscles
84 S. Tanaka Journal of Insect Physiology 47 2001 83–94
cannot fly but reproduces earlier and in many species is more fecund than the LW morphs Harrison, 1980; Din-
gle, 1985; Roff, 1986; Zera and Denno, 1997. The gen- eral consensus is that the absence of the large wings and
functional flight musculature allows more energy to be devoted to reproduction and survival Dingle, 1985.
Therefore, it is reasonable to assume that there is an energetic cost involved in the development and mainte-
nance of the flight capability in the migratory morph. This idea is based on an assumption that resources are
limited within the organism, and thus trade-offs occur between different life-history traits Calow 1978, 1979;
Stearns, 1992. However, the presence of a correlation between wing morph and reproductive characteristics
does not necessarily prove their causal relationship Tanaka, 1993; Tanaka and Suzuki, 1998.
To address issues concerning flight capability in wing dimorphic insects, physiological studies are necessary.
Mole and Zera 1993, 1994 have compared nutritional indices and flight muscle development profiles between
LW and SW morphs of the crickets Gryllus rubens and G. firmus to identify potential physiological costs of
flight. Their main findings are as follows. 1 There was no direct trade-off between flight muscle and ovarian
growth. 2 There was no intermorph difference in food consumption in G. rubens, but LW morphs ate more
food than SW morphs in G. firmus. 3 LW morphs con- verted a lower proportion of assimilated nutrients into
biomass i.e. mainly the ovary than did SW morphs. 4 Ovarian growth was greater in SW morphs than in LW
morphs in G. rubens, but no difference was found in G. firmus. 5 No intermorph difference was found in the
number of eggs laid in either species, in contrast to pre- vious results obtained for these Roff, 1984; Zera and
Rankin, 1989 and other species Tanaka 1976, 1993. These results, together with those reported by Zera et
al. 1994, provided the basis for the trade-off theories discussed in more recent articles by Zera and his col-
leagues Zera and Denno, 1997; Zera et al., 1998.
It seems surprising that the closely related species of G. rubens and G. firmus show such physiological incon-
sistency. In fact, some of the findings related to fecundity and flight muscle development contradict earlier ones
with the same and other species, as pointed out by Crno- krak and Roff 1998, Tanaka and Suzuki 1998 and
Tanaka et al. 1999. In crickets, it has been known that ovarian development and food consumption are influ-
enced by various factors such as temperature, mating, and the presence or absence of oviposition substrate
Loher and Edson, 1973; Clifford and Woodring, 1986; Loher et al., 1987; Renucci et al., 1990. One of the
consistent results in the studies by Mole and Zera 1993, 1994 is the unusually low ovipositing activity in spite
of the fact that all females were mated; in the first two weeks of adult life, G. firmus females produced only 83
eggs on average and G. rubens females only 20 eggs. As pointed out by Tanaka and Suzuki 1998, this is in
marked contrast to .400 for G. firmus reported by Roff 1990 and .250 reported for G. rubens by Zera and
Rankin 1989. In the present study, I conducted experi- ments on egg retention to mimic this low egg-laying in
a cricket. In a more recent study, Zera et al. 1994 reported intermorph differences in lipid and carbo-
hydrate contents in G. firmus, but all test females were not allowed to lay eggs during the entire period of the
experiment. The possible impact of suppressed ovi- positing activity on various physiological traits as well
as on trade-offs was pointed out by Tanaka and Suzuki 1998 and Tanaka et al. 1999, but no study has been
designed to examine these specific problems.
In the present study, I investigated the effects of sup- pression of oviposition activity on food consumption,
ovarian development and body weight in female adults of a wing dimorphic cricket, Modicogryllus confirmatus.
Because food consumption and ovarian weight are known to be influenced by the development and histoly-
sis of the flight muscle in LW morphs of this species Tanaka, 1993; Tanaka et al., 1999, a group of LW
morphs in which flight muscle histolysis was induced by artificial de-alation at adult emergence Tanaka, 1994a
was included to determine the role of flight muscle his- tolysis in the control of food consumption under stressful
conditions. In crickets, flight muscle histolysis is com- mon in LW morphs, and in several species LW females
with histolyzed flight muscles develop their ovaries as rapidly as SW females Tanaka 1976, 1986; Tanaka,
1994a,b; Zera et al., 1997. The results of these obser- vations are reported in this paper. They strongly indicate
that suppression of ovipositing activity and failure to recognize the occurrence of flight muscle histolysis
could 1 partly explain the above-mentioned sporadic results reported by Zera and his colleagues Mole and
Zera 1993, 1994; Zera and Mole, 1994; Zera et al., 1998, and 2 call into question the validity of the bio-
chemical comparison between wing morphs in crickets deprived of oviposition substrate Zera et al., 1994.
Hence some of the generalizations made by Zera and Mole 1994, Zera and Denno 1997 and Zera et al.
1998 may also need re-evaluation.
2. Materials and methods
