Ecological Entomology (2017), DOI: 10.1111/een.12471 Estimating consumable biomass from body length and order in insects and spiders SAMANTHA STRAUS and L E T I C I A AVILÉS Department of Zoology, University of British Columbia, Vancouver, Canada Abstract. 1. Current models used to estimate insect prey biomass for diet studies use whole weight. However, a large proportion of an arthropod’s body is taken up by an indigestible exoskeleton, leading to erroneous estimation of the food intake of insectivorous animals. 2. Linear mixed effect models were used to obtain equations to predict consumable biomass from body length for a variety of Neotropical insects and spiders. These data were obtained by feeding taxa of various orders to groups of 100 social spiders and comparing pre- and post-consumption weights using size-matched controls. 3. Significant linear relationships were found relating body size to consumed biomass for all orders, with slopes ranging from 1.276 to 4.011 and R 2 values from 0.476 to 0.929. For orders other than spiders and Orthoptera, the increase in weight with size exhibited negative allometric scaling, suggesting a decrease in tissue density, or an increase in internal air space, with size. 4. Although there were significant differences across taxonomic orders in the proportion of biomass consumed, within most orders the proportion consumed did not differ significantly with body size. The estimated regression coefficients may be used by other workers to estimate consumable biomass of arthropod prey for studies requiring large sample sizes or non-lethal sampling of rare or endangered species. Key words. Allometric scaling, insect length–weight scaling, predation, scaling coef- ficients, social spiders, tropical rainforest insects. Introduction Understanding the proportions of digestible and indigestible material in insects should be an important consideration when estimating the nutritional intake of insectivorous animals. How- ever, most often, researchers use whole-insect dry biomass as a proxy for food intake. Sage (1982) published an influential study estimating dry biomass from length for terrestrial insects. Recently, Sage’s equations were used to quantify insect biomass consumed by lizards by identifying chitinous insect carcasses in lizard faeces (Perez-Cembranos et al., 2016). Another recent study used dry insect biomass to estimate food intake in social spiders (Yip et al., 2008). However, a large proportion of that dry biomass is taken up by indigestible materials (i.e. chitin), with the proportion of indigestible material potentially differ- ing among taxonomic groups with different body plans. Perkins et al. (2013), for instance, showed differences in the proportion Correspondence: Samantha Straus, Department of Zoology, Uni- versity of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada. E-mail: straus@zoology.ubc.ca of digestible and indigestible tissues of aphids and predatory hoverflies by comparing stable isotope signatures in exoskele- ton and soft tissues. Ignoring taxa-specific differences in the proportions of digestible and indigestible tissues may result in researchers erroneously estimating the dietary and energetic intake of insectivorous animals. The objective of this study was to estimate the proportion of digestible and indigestible biomass for different taxonomic cat- egories of arthropod prey and obtain order-specific equations to estimate total and consumable biomass from body length. For the purposes of prey digestion, we used group forag- ing spiders, as, through cooperative prey capture and feeding, these organisms are able to extract digestible biomass from a large range of insect sizes (Yip et al., 2008). Spiders have extra-oral digestion, whereby digestive enzymes are injected into prey to break down organs and the extracellular matrix that binds them (Cohen, 1998; Salomon et al., 2008; Fialho et al., 2012). Digestible parts are then extracted, leaving behind indigestible parts (Edgar, 1971; Wilder et al., 2013). Other predators, which are not social spiders, may have different © 2017 The Royal Entomological Society 1