Body Size

The general public has always been fascinated with "giant" cockroaches. Discoveries of large species, whether alive or in the fossil record, are thus guaranteed a certain amount of attention. The concept of body size, however, is qualitative and multivariate in nature (McKinney, 1990). Consider two cockroaches that weigh the same but differ in linear dimensions. Is a lanky, slender species bigger than one with a stocky morphotype? Neotropical Megaloblatta blaberoides (Nyctiborinae) triumphs for overall length (head to tip of folded wing) (Fig. 1.5). The body measures 66 mm, and when the tegmina are included in the measurement, its length tops out at 100 mm. This species has a wingspan of 185 mm (Gurney, 1959), about the length of a new pencil. Also in contention among the attenuated, lighter-bodied cockroaches are several in the oft-cultured genus Blaberus. Blaberus giganteus may measure 80 mm overall (60 mm body length) and female Blab. craniifer 62 mm. Pregnant females of the latter weigh about 5 g (Nutting, 1953a). A male Archimandrita tessalata measured by Gurney (1959) stretched to 85 mm, and one of the largest species in West Africa (more than 60 mm) is Rhyparobia (= Leucophaea) grandis (Kumar, 1975). Recently, a large cockroach in the genus Miroblatta was discovered in caves and rock shelters in limestone formations in East Kalimantan, the Indonesian section of Borneo.1 The cockroach was widely reported as being 100 mm in length (e.g., BBCNews, 23 December 2004). Two males measured by Drs. Anne Be-dos and Louis Deharveng were 60 mm, but they noted that some specimens, particularly females, may be larger. The cockroach is a streamlined, long-legged species that moves very slowly on tiptoe, with the body elevated up over the substrate. It is a beautiful reddish-brown, with lighter-colored legs and wings that are about half the length of the abdomen.

In the heavyweight division, the undisputed champs are the wingless, burrowing types. The Australian soil-burrowing behemoth M. rhinoceros weighs in at 30 g or more, and can measure 85 mm in length. Macropanesthia rothi is sized similarly to M. rhinoceros, but is more robust in the thorax and legs (Rugg and Rose, 1991; Walker et al.,

1. For information on the species, we thank Patricia Crane, Leonardo Salas, Scott Stanley, and Louisa Tuhatu of the Nature Conservancy, and Louis Deharveng, Anne Bedos, Yayuk Suhard-jono, and Cahyo Rachmadi, the entomologists in the expedition that discovered the species. The cockroach was identified by P. Grandcolas.

1994). Males of Macropanesthia are frequently mistaken for small tortoises during periods of surface activity (Rentz, 1996). The Malagasian G. portentosa can reach 78 mm in length (Gurney, 1959), and G. grandidieri, with a body length of 85 mm, rivals M. rhinoceros in size (Walker et al., 1994).

The oft-repeated myth that the Carboniferous was the "Age of Giant Cockroaches" is based on the size of fossil and modern cockroaches that were known during the late 1800s. More recently described species of extant cockroaches raise the modern mean, and scores of recently collected small fossil species will no doubt lower the Paleozoic mean (Durden, 1988). The fossil record also may be biased in that large organisms have better preservation potential, are easier to find, and can better survive incarceration in fine- and coarse-grained sediments (Carpenter, 1947; Benton and Storrs, 1996). Small cockroaches, on the other hand, may be filtered from the fossil record because they are more likely to be swallowed whole by fish during transport in flowing water (Vishniakova, 1968). The largest fossil cockroach to date is an undescribed species from Columbiana County, Ohio, which has a tegmen length of at least 80 mm (Hansen, 1984 in Durden, 1988); a complete fossil from the same location has recently received media attention (e.g., Gordner, 2001). Nonetheless, the tenet that no fossil cockroach exceeds in size the largest living species (Scudder, 1886; F.M. Carpenter in Gurney, 1959) still applies. It would not be unreasonable to suggest that we are currently in the age of giant cockroaches (C. Durden, pers. comm. to CAN)!

At the other end of the scale, the smallest recorded cockroaches are mosquito sized species collected from the nests of social insects, where a minute body helps allow for integration into colony life. The myrmecophile At-taphila fungicola is a mere 2.7 mm long (Cornwell, 1968) (Fig. 1.5), and Att. flava from Central America is not much larger—2.8 mm (Gurney, 1937). Others include Myrmecoblatta wheeleri from Florida at less than 3 mm (Deyrup and Fisk, 1984), and Pseudoanaplectinia yumo-toi (4 mm) from Sarawak (Roth, 1995c). Australian species of Nocticola measure as little as 3 mm and have been collected from both termite nests and caves (Rentz, 1996). Another category of cockroaches that can be quite small are those that mimic Coleoptera. Plecoptera poeyi, for example, lives on foliage of holly (Ilex) in Florida and is 5-6 mm long (Helfer, 1953). To put the sizes of these cockroaches into perspective, it is worthwhile to note that the fecal pellets of M. rhinoceros are 10 mm in length (Day, 1950).

As a group, blattellids are generally small in size, but several genera are known to include moderately large members (Rentz, 1996). A number of tiny aerial Blattel-lidae live in the canopy of tropical rainforests, where

"their size is suited to hiding in the crease of a leaf or by a small bit of moss" (Perry, 1986). Small bodies may confer a survival advantage in graduate student lounges; Park (1990) noted that American cockroaches live for about 5 sec when placed in a microwave oven set on "high," but the more diminutive German cockroach lasts for twice that long. Small cockroaches usually mature more rapidly and have shorter lives than the larger species (Mackerras, 1970).

Intraspecific variation in cockroach body size can be considerable, with the difference between the largest and the smallest specimens so great that they appear to be different species (Roth, 1990b). Male length in Laxta granulosa, for example, ranges from 14.8 to 25.4 mm (Roth, 1992). In most cockroaches, the abdominal segments can telescope. Extension of the abdomen in live specimens and shrinkage in the dead ones, then, may contribute to noted variation when body length is the measurement of choice. Body size may vary within (e.g., Platyzosteria melanaria—Mackerras, 1967b), and between (e.g., Par-coblattini—Roth, 1990b), geographic locations, or be rather consistent over an extensive range (e.g., Ectobius larus, E. involutus—Rehn, 1931). No latitudinal clines in body size have been reported in cockroaches.

As in most invertebrates (Fairbairn, 1997; Teder and Tammaru, 2005), sexual dimorphism in body size of adult cockroaches is common. All patterns are exhibited, but a female size bias seems to predominate (Fig. 1.6). Examples include Colapteroblatta surinama, where females are 18.5-19.0 mm and males are 13.0-15.5 mm in length (Roth, 1998a), and the cave-adapted species Trogloblat-tella nullarborensis, with females measuring 34.5-38.5 mm and males 24-27.5 mm (Roth, 1980). Because of in-traspecific variation and the multivariate nature of size, however, generalizations can be difficult to make. Males may measure longer than females, especially when wings are included in the measurement, but females are usually broader and bulkier, particularly in the abdomen. Both P americana and Supella longipalpa fall into this category (Cornwell, 1968) (Fig. 1.7). Several burrowing cockroaches exhibit little, if any size dimorphism. There is no significant difference in the fresh weight or head capsule width of males and females of field-collected pairs of Cryptocercus punctulatus, but the dry weight of females is slightly higher (Nalepa and Mullins, 1992). In most Geoscapheini, males and females are of similar size (Fig. 1.8) (e.g., Walker et al., 1994), as are several species of Sal-ganea, such as Sal. amboinica and Sal. rugulata (Roth, 1979b). In some Salganea, however, the male is distinctly smaller than the female. These include Sal. rectangularis (Roth, 1999a) and Sal. morio, where males average 41.9 mm in length and females 46.6 mm (Roth, 1979b). Species in which males outsize females include several

Fig. 1.6 Diagrammatic representation of cockroach species showing comparative size, comparison between males (left) and females (right), degree of size variation within a sex (minimum measurement on left, maximum measurement on right), and relationship between tegmen and body length. From Cornwell (1968),based on data from Hebard (1917). With permission ofRen-tokil Initial plc.

Fig. 1.6 Diagrammatic representation of cockroach species showing comparative size, comparison between males (left) and females (right), degree of size variation within a sex (minimum measurement on left, maximum measurement on right), and relationship between tegmen and body length. From Cornwell (1968),based on data from Hebard (1917). With permission ofRen-tokil Initial plc.

Parcoblatta species (Fig 1.6) (Pare. lata, Parc. bolliana, Parc. divisa, Parc. pennsylvanica). Males of the latter are 22-30 mm in length, while females measure 13-20 mm. In Parc. fulvescens, however, females outsize the males (Cornwell, 1968; Horn and Hanula, 2002).

Like other animals, the pattern of sexual size dimorphism within a cockroach species is related to the relative influence of body size on fecundity in females and mating success in males. In G. portentosa, males tend to be larger than females, and big males are the more frequent victors in male-male contests (Barth, 1968c; Clark and Moore, 1995). In species where males offer food items to the female as part of courtship and mating, nuptial gifts may reduce the value of large size in females and increase its value in males (Leimar et al., 1994; Fedorka and

Mousseau, 2002). This hypothesis is unexplored in the cockroach species that employ such a mating strategy. One proximate cause of female-biased sexual size dimorphism in cockroaches is protandry. Males may mature faster than females because it gives them a mating advantage, but become smaller adults as a consequence. Males of Diplopterapunctata, for example, usually undergo one fewer molt than do females, and require a shorter period of time to mature (Willis et al., 1958). Males of Aniso-gamia tamerlana mature in five instars, and females in six (Kaplin, 1995).

Physiological correlates of body size have been examined in some cockroaches; these include studies of metabolic rate and the ability to withstand extremes of temperature, desiccation, and starvation. Coelho and Moore

Fig. 1.7 Male (left) and female Supella longipalpa, showing dissimilarity in form between the sexes. The female is stouter, and the head is broader with a larger interocular space; the pronotum is also larger than that of the male. The tegmina of the female reach only to the end of the abdomen and are more chitinous than those of the male (Hebard, 1917). From Back (1937), with permission from the Entomological Society of Washington.

Fig. 1.7 Male (left) and female Supella longipalpa, showing dissimilarity in form between the sexes. The female is stouter, and the head is broader with a larger interocular space; the pronotum is also larger than that of the male. The tegmina of the female reach only to the end of the abdomen and are more chitinous than those of the male (Hebard, 1917). From Back (1937), with permission from the Entomological Society of Washington.

(1989) found that resting metabolic rate for 11 species scales allometrically (VO2 = 0.261 M °.?76) with mass. As in other animals, then, it is metabolically more expensive for a small cockroach to maintain a gram of tissue than it is for a large one. Relative brain size has been compared

Fig. 1.8 Harley A. Rose, The University of Sydney, displaying male-female pairs of Australian soil-burrowing cockroaches (Geoscapheini). Photo by C.A. Nalepa.

in two cockroach species. The brain (supra-esophageal ganglia) of B. germanica occupies about 10 times as much of the cranial cavity as does that of M. rhinoceros, a species that weighs 320 times more (Day, 1950) (Fig 1.9). There is, however, no marked difference in the size of individual nerve cell bodies. Day thought that the large size of Macropanesthia could be attributed to its burrowing habit, which "greatly reduces the effectiveness of gravity in limiting size." More likely factors include the ability to withstand predation, the power required to dig in indurate soils, and the lower rate of water loss associated with a small surface to volume ratio. The latter was suggested as being influential in G. portentosa's ability to thrive in the long tropical dry season of Madagascar (Yo-der and Grojean, 1997); in the laboratory adult females survived 0% humidity without food and free water for a month.

The social environment experienced during development influences adult body size in cockroaches. Isolated cockroach nymphs mature into larger adults than nymphs that have been reared in groups, but a smaller adult body size occurs when nymphs are reared under crowded conditions (e.g., Willis et al., 1958; Woodhead and Paulson, 1983). Unlike laboratory studies, however, overpopulation in nature may be relatively rare, except perhaps in some cave populations. Crowded adults are likely to disperse or migrate when competition for food and space becomes fierce. In all known cases where biotic or abiotic factors affect cockroach adult size, these factors act by influencing the duration of juvenile growth. In D. punc-

Fig. 1.9 Comparison of the relative size of the head and anterior nervous system in (A) Macropanesthia, and (B) Blattella. From Day (1950), with permission from CSIRO Publishing.

tata, the greater adult weight of isolated animals results from a longer nymphal development. Males normally have three or four instars, but isolation results in a higher proportion of the four-instar type (Woodhead and Paulson, 1983). A longer postembryonic development induced by suboptimal diet resulted in heavier adults in Blaptica dubia (Hintze-Podufal and Nierling, 1986). In three families of Cryptocercus clevelandi monitored under field conditions, some of each litter matured to adults a year before their siblings did. Those that matured in 6 yr had larger head widths than those that matured in 5 (Nalepa et al., 1997).

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