Claudia Cressa ¹

Received 22-I-1998. Corrected 16-X-1998. Accepted 10-XI-1998.

Abstract

Relationships of body mass and head capsule width were developed for Thraulodes sp., Haplohyphes sp. (Ephemeroptera), Leptonema sp., Phylloicus sp. Nectopsyche sp. (Trichoptera), and Anacroneuria sp. (Plecoptera). The organisms were collected with a Surber net (0.1296 m² and 0.286 mm mesh size) on the Orituco river, Venezuela. The number of specimens used in the analysis for the species studied, was different (from 84 to 103 specimens). Regression analysis indicated that all relationships were best expressed by a power function rather than by linear or exponential equation. Analyses of the relationships reveal interspecific differences among insects of the same order. Species-specific relationships are recommended to be used whenever possible since, depending on the species, the underestimation of mass could be as much as 56%.
 

Key words

Biomass, length-mass relationships, aquatic insects, tropics
 

It is recognized that biomass estimates are an essential methodological aspect in order to determine growth rate and/or production of aquatic macroinvertebrates as well as to understand life histories and trophic relationships between functional feeding groups (Benke 1996). However, benthic biomass could also be used in studies of colonization rates or in the quantification of the role that macroinvertebrates play in the decomposition of leaves. In any of these studies, two main problems are encountered. First, the high number of samples that must be taken to overcome spatial heterogeneity, when sampling a river, precludes the attempt to separate, identify and weight animals while still alive (Cressa 1986). Secondly, when studying decomposition rates where shredders with cases are numerous, they have to be separated from their cases to determine their biomass. This is a time consuming process and the manipulation error (breaking of body structures) is high due to the fragility of these organisms. Thus, to overcome these problems it is necessary to make indirect dry biomass determinations, of which the simplest method is to measure live biomass (Parma 1971). Nevertheless, this is not a suitable method, since differences on live mass of aquatic organisms might reflect not only differences in biomass, but differences on water retention (Dermott & Paterson 1974, Schema et al. 1981, Traina & Von Ende 1992, Dumont & Balvay 1979). Therefore, direct determination of dry mass is recommended in order to obtain reliable estimates of biomass (Donald & Paterson 1977, Sudgen 1967, Slack 1967, Stanford 1973, Mason et al. 1983, Meyer 1989). ]]> Measuring any body structure of an insect is less time-consuming than to weight all the organisms collected. Since fundamental relationships do exist between linear body measurements (body length, head width, head length) and biomass (Gould 1966, Peters 1983), they have been used in order to obtain indirect biomass estimates. Wenzel et al. (1990) indicated that a difference of 20% could be expected between actual and theoretical biomass.

Even though the importance of length-dry mass relationships is well known, few data are available for aquatic macroinvertebrates living in temperate habitats (Clifford 1970, Cianciara 1980, Meyer 1989, Smock 1980, Wenzel et al. 1990, Burgherr & Meyer 1997) and even fewer for aquatic as well as terrestrial tropical insects (Lewis 1975, Cressa & Lewis 1984, Cressa 1986, López et al. 1996-1997). Furthermore, the data available suggest the need to use taxa-specific regressions since they are more accurate (Schoener 1980, Smock 1980, Cressa 1986, Gowing & Recher 1985). The objective of this work is to increase this knowledge providing length-specific mass relationship for some common species of tropical insects: two species of Ephemeroptera (Thraulodes sp. and Haplohyphes sp.), three of Trichoptera (Leptonema sp. Phylloicus sp. and Nectopsyche sp.) as well as a species of Plecoptera (Anacroneuria sp.).
 

Materials and Methods

The basic data sets used for calculation were derived from a study on community structure, standing crop and secondary production of macroinvertebrates in the Orituco River, Venezuela (9°57 -10° 1’ N, 66°;24’ - 66°26’ W). A description of the river as well as its physicochemical characteristics is given in Cressa & Senior (1987).

The taxa chosen for this study are among the most important representatives of the benthos community in rivers of Venezuela (Cosme 1985, Cressa 1994, 1997, Rincón 1996). In this particular stream, they comprise 64.2 % of the macroinvertebrate community. On the other hand, species were selected because they were the most abundant at the order level (Cressa 1994).

Larvae were collected with a Surber net (0.1296 m² and 0.286 mm mesh size), anesthetized in the field with CO₂ and placed in a cooler for transportation to the laboratory where they were kept at 5 °C. Samples were examined within 48 h after collection and dry mass was determined individually on animals that had been anesthetized but not preserved, as it is known that preservation causes losses of dry mass by leaching (Dermott & Paterson 1974, Donald & Paterson 1977, Mason et al. 1983, Benke 1996).

In the laboratory the larvae were cleared of attached detritus particles, identified and head capsule width determined to the nearest 10 µm with a stereomicroscope fitted with an ocular micrometer. Head capsule width was measured as the distance across the widest portion of the head. Subsequently, the animals were dried at 60 °C during 24 hours. After cooling in a desiccator for 24 h, they were weighed to the nearest 10 µg with an electrobalance. Individual animals were weighed one at the time except for the smallest size of Nectopsyche sp. and Haplohyphes sp. For these taxa animals with same head capsule width were pooled (2-3), and the mean weight determined. The data reported represents actual number of animals used for the statistical analysis (Table 1).
 

]]>

			Head capsule width (µm)					Dry weight (µg)
Taxon	]]> n	Range			S	cv	Range			s	cv
Ephemeroptera
Thraulodes sp.	98	]]> 108.0 - 2450		818.62	515.18	62.93	5.10 - 8962.69		1694.78	2086.06	123.09
Haplohyphes sp.	94	66.5 - 962		]]> 393.80	217.10	55.13	12.00 - 836.02		208.59	168.75	80.90
Trichoptera
Leptonema sp.	85	152.0 - 1875		723.69	]]> 345.60	47.76	9.10 - 16443.84		1689.26	3166.66	187.46
Nectopsyche sp.	103	72.0 - 705		301.23	131.71	]]> 43.72	15.81 - 383.24		108.25	72.54	67.01
Phylloicus sp.	87	238.7 - 1258		703.75	327.12	46.48	]]> 6.63 - 10167.00		2857.21	3339.42	116.88
Plecoptera
Anacroneuria sp.	84	375.0 - 2812		1060.86	527.82	49.74	131.80 - 11071.22		]]> 1406.41	2055.06	120.14

cv = coefficient of variation in %: (s /  ) x 100

Some authors have indicated that body length is a better predictor to estimate biomass than head capsule width (Smock 1980, Meyer 1989, Burgherr & Meyer 1997). However, since the objective of this study was to develop a fast and reliable method to estimate biomass, body length was not used because among other reasons the head capsule is a heavily sclerotized structure, not subject to distortion or breakage under manipulation as is body length.

Trichoptera are usually collected with their cases and it was thought that it would be interesting to determine if some relationship exists between some dimensions of the case and body weight. Therefore, the width (widest part at the opening) and weight of the case were determined for Phylloicus sp. and Nectopsyche sp. If the resulting relationship is as good as the relationship between head capsule width and body mass it would allow easier determination of growth rate in the laboratory or field experiment since manipulation could be minimized.

Regression analyses were used to determine the relationship between linear measurements (head capsule width) and dry weight. The data were fitted to the following models in order to determine which best described the width-weight relationship of the various species: power (y = axb), exponential ( y = aebx) and linear (y = a +bx) models, where y is weight (µg), x is head capsule width (µm) and aand b are regression constants. The analyses were performed on logarithmic transformed data (log10), to express the exponential and power equations in linear form.
 

Results

In general, the range of weight is higher than that of head capsule width among the different species, with Leptonema sp., Thraulodes sp. and Phylloicus sp. showing the highest coefficients of variation (Table 1). ]]> Head capsule width and weight were highly correlated for all species (Table 2, p < 0.001), with the power model best describing this relationship. This assertion was corroborated with the analysis of residuals for each equation using studentized residual plots, which indicated that none of the regression model assumptions were violated. Similar results have been found for terrestrial (Roger et al. 1976, Gowing & Recher 1984) and for aquatic insects (Smock 1980, Wenzel et al. 1990, Meyer 1989, Burgherr & Meyer 1997). Since Nectopsyche sp. was the only species where the coefficient of determination for the linear equation (0.852) was almost the same than for the power equation (0.871), the data were analyzed using the power equation and the results are summarized in Table 2.

^{* Regression equation for the relationship between case width (m m) and dry mass (m g)
a, b = regression constants, SE = standard error of the estimate, r2 = coefficient of determination.}

]]> The values of b are lower than the expected value of 3 predicted by the surface law (Hill 1976, Gould 1966, Smock 1980, Wenzel et. al. 1990), with the exception of Phylloicus sp. (4.49, Table 2), which has the highest value. The equations indicate a high difference in intercepts for the different species, reflecting their different average sizes (Table 1). Change in body weight per unit change in head capsule width differs greatly among species from the same insect order. Unplanned comparison of the regression coefficients for the different species (GT2, p < 0.01, Sokal & Rohlf 1981) indicates that all slopes were significantly different from each other with the exception of the comparison between Haplohyphes sp. and Nectopsyche sp. (GT2, p 0.05, Sokal & Rohlf 1981).

Predictive equations for each insect order was calculated from pooled data for individuals in that particular order and are also given in Table 2. All relationships were highly significant (p < 0.001) but the fit at order level is poorer than at lower taxonomic level (Table 2). Pairwise comparison of these resulting slopes indicated a nonsignificant difference between Plecoptera and Ephemeroptera (GT2, p 0.05, Sokal & Rohlf 1981), suggesting a similar relationship between body mass and head capsule width for these taxa. When Phylloicus sp. is not considered in the pooled data of Trichoptera, the pairwise comparison of the slopes for the three orders are nonsignificant different from each other.

A predictive general equation was also obtained by pooling data from all individuals measured in this study (Table 2) which will be used for comparison with published data (Table 3).
 

Case width and dry mass of caddisfly larvae were highly correlated (p < 0.001) for Nectopsyche sp.and Phylloicus sp. even though their coefficients of determination were lower than when using body measurements (Table 2). Comparison of slopes for Nectopsyche sp. indicates a nonsignificant difference (GT2, p 0.05, Sokal & Rohlf 1981), suggesting that changes in body weight per unit change in case width or head capsule width is similar. On the other hand, comparison of slopes for Phylloicus sp. indicate a significant difference (GT2, p < 0.05, Sokal & Rohlf 1981), suggesting that changes in body weight per unit change in case width are smaller than changes in body weight per unit change of head capsule width.
 

Discussion

The length-mass relationships are useful since they allow a fast determination of biomass when processing a high number of samples. The data presented here, although based on a few taxa, reinforce the notion that in tropical aquatic insects the constant b falls short of the expected value of 3. The underlying assumption for this value is that body mass of insects is more influenced by surface than by volume (Engelmann 1961). Thus, it is possible that tropical aquatic insects could have longer thinner bodies than those in temperate habitats, as was suggested by Schoener (1980) when working with terrestrial tropical insects. Since Phylloicus sp. was the only species whose slope is higher than 3, it will be interesting to compare body length-dry mass relationship of this species from temperate habitats with tropical ones. Furthermore, it is possible that in this genus volume could influence body mass more than surface as is suggested by the data when. Phylloicus sp. and Leptonema sp. are compared (see Table 1) since the former is wider and shorter than the latter.

On the other hand, Trichoptera seems a particularly interesting order for further comparisons since the difference in slopes among species was the greatest. Even though the number of species analyzed here are few, the range in slope values is also higher (Table 2) than the one shown by Meyer (1989) with a bigger data set (2.86 - 3.64, head capsule width and dry mass relationships).

Only few comparable results are available which might serve as a basis for evaluating the information presented here. Thus, a summary of the data already published is presented in Table 3. The values indicate that head capsule width-dry mass relationships are similar at the order level for insects from North American and Central Europe. For the tropical species analyzed in this study, with the exception of Phylloicus sp., as well as for Ephemera (E.) spilosa (Dudgeon 1996), the data indicates that changes in weight are smaller than in head width. However, since there are few data available, it is impossible to indicate if tropical insects in general will have lower slopes (b < 3) than temperate ones. This hypothesis should be supported by further studies comprising a higher number of taxa.

A relationship between weight and case width was expected, since a change in case dimensions is needed as animals grow. Surprisingly though, this correlation was as high as for the relationship between weight and head capsule width. This result will be useful in growth rate studies because of the following advantages: (i) the same animal could be subject to measurements throughout the experiment, (ii) it will eliminate data extrapolation since measurements will belong to the same individual and (iii) it will avoid the use of a high number of experimental animals, since samples would not have to be subdivided, and therefore lost, in order to determine biomass at particular intervals.

The regression equations and relationships presented here are suitable for investigations into production biology since the results of several samples are used to estimate total biomass of the species under study. ]]> Furthermore, the data also illustrate the need to use species-specific equations since their predictive capability is higher than when using equations based on data grouped at higher taxonomic levels. In order to illustrate this, I have computed the predicted weight of an individual of each one of the species studied from three different equations: (i) general (all data pooled), (ii) order-specific and (iii) species-specific. The linear measurements from which the weight predictions are made represent the midpoint of the range for the species. The predicted weights are shown in Table 4, from which it is evident that errors associated with substitution of the species-specific equations for the general or order-specific equations may lead to underestimation of biomass from 5.7 % to as much as 82%, with Phylloicus sp. being the species most affected. On the other hand, the overestimation of weight by using the order specific or the general equation was not as high (12% - 43%). The highest overestimation of mass was for Anacroneuria sp. when using the general equation.
 

Acknowledgements

This study has been supported by the Consejo de Desarrollo Científico y Humanístico of the Universidad Central de Venezuela (Grant No. C03-10-2280/90). I am grateful to Pedro Hernández Gil for allowing access to its private land, to Brunilda Heredia, Director of the Guanapito Experimental Station (FONAIAP), for her logistic support, to Belkis Pérez for general assistance and to the Institute of National Parks for allowing use of its facilities. The comments of Peter Burgherr on an earlier version of the manuscript are greatly appreciated.
 

]]> Resumen

Se calcularon las relaciones entre la masa del cuerpo y el ancho de la capsula cefálica para Thraulodes sp., Haplohyphes sp. (Ephemeroptera), Leptonema sp., Phylloicus sp., Nectopsyche sp. (Trichoptera), y Anacroneuria sp. (Plecoptera). Los análisis de regresión indicaron que todas las relaciones se expresan adecuadamente mediante la función potencial en vez de la función exponencial o linear. El análisis de las relaciones obtenidas revela diferencias entre los insectos pertenecientes a un mismo Orden. Se recomienda el uso de relaciones específicas para cada especie siempre y cuando sea posible, debido a que, dependiendo de la especie, la subestimación de la masa puede llegar a ser hasta de un 56%.
 

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¹ Instituto de Zoología Tropical, Facultad de Ciencias, Universidad Central de Venezuela, Apartado Postal 47058, Caracas 1041-A, Venezuela. Fax: 58-2-7535897. E-mail: ccressa@strix.ciens.ucv.ve ]]> 1996 557-578 1997 139 101-112 1980 27 377-406 1970 48 305-316. 1985 1986 37 170-173 1994 25 1853-1855 1997 26 1984 100 21-28 1987 38 99-105 1974 52 1243-1250 1977 53 75-80 1996 135 473-483. 1979 64 139-145 1961 31 221-238. 1966 41 587-640 1984 9 5-8 1985 10 195-0 1976 1996 -1 99 45/46 641-643. 1975 1983 3 29-44 1989 117 191-203 1971 1983 2. 1996 1976 6 387-389 1980 73 106-109. 1981 . 100 326-329 1967 24 1017-1033. 1980 10 375-383 1981 1973 54 449-451 1967 12 1992 228 219-223 1990 118 31-46