Constant seed size and mandible growth – a fundamental problem for granivorous ground beetle larvae (Coleoptera: Carabidae)

Abstract

Feeding on small tree seeds at fruit falls is a specific adaptation of harpaline ground beetles and their larvae in tropical rain forests. Using mandibles as tools, they have to perforate the seed shell to reach the nutritious interior. The isometric growth of larval mandibles, known from predatory species, would result in a changing ratio between seed and mandible size during the course of development. The stable size of host tree seeds should select an optimum mandible size, similar for the three larval instars in spermophageous species. We found an increasing tendency to maintain the length of the apical mandible part (apex) in seed feeding species.The size increase is higher in the species from Australia, Africa and South East Asia than in the New World species of the genus Notiobia. Feeding experiments have revealed that larvae of species with a stronger increase in apex growth are also able to develop as predators of Drosophila larvae. Our results furthermore point to an influence of beetle size and shell hardness of the host tree seeds on the apex size and its growth rate.

Granivory in terrestrial isopods

“…Seeds were thus an attractive food source for this species of isopod and granivory was not an artefact of starvation. This is very similar to the situation in Carabidae, where the gape of the mandibles determines the sizes of the prey/seed that they can feed on ( Wheater, 1988;Paarmann et al , 2006 ). Since isopods select food of high quality ( Rushton & Hassall, 1983, 1987Tuck & Hassall, 2004 ), the results of this study indicate that seeds may be suitable food for isopods.…”

“…The other species of seed tested in this study were small enough to be processed by the mandibles of a wide range of isopod size classes. This is very similar to the situation in Carabidae, where the gape of the mandibles determines the sizes of the prey/seed that they can feed on ( Wheater, 1988;Paarmann et al , 2006 ). Size constraints to seed predation thus seem to be general for invertebrate seed predators.…”

The Role of Ground Beetles (Coleoptera: Carabidae) in Weed Seed Consumption: A Review

“…These preferences are mainly expressed by the early larval instars and might be governed by morphological traits such as the size of the larval head capsule. For example, first-instar larvae of Amara similata fed exclusively on seeds of common chickweed (Paarmann et al 2006) and shepherd's purse (Klimes and Saska 2010), whereas the later instars had generalist feeding habits that lacked discrete preferences. First-instar larvae of the generalist species, Amara aenea, had more difficulty in crushing the hard seed coat of dandelion compared with the late instars, which supports the idea that morphological constraints are an important factor in the seed preferences of granivores (Paarmann et al 2006).…”

Utility of geometric morphometrics for inferring feeding habit from mouthpart morphology in insects: tests with larval Carabidae (Insecta: Coleoptera)

“…The focal similarities were also confirmed by geometric morphometrics. To my knowledge, this is the first report on quantitative evaluation of ontogenetic change in mandible shape in carabid larvae [note that, although Paarmann et al (2006) also examined ontogenetic changes in the mandible, they focused on size rather than shape]. The phenomenon that the overall shape is determined by coordinated variation among subunits is a typical case of morphological integration, which, together with modularity, is an important concept for understanding the mechanisms of morphological diversification of organisms (Zelditch et al, 2012).…”

“…For example, in RW2 of data set 1, log centroid size had a large effect size and differences in this axis between the larval instars were recognized in most species. To my knowledge, this is the first report on quantitative evaluation of ontogenetic change in mandible shape in carabid larvae [note that, although Paarmann et al (2006) also examined ontogenetic changes in the mandible, they focused on size rather than shape]. Currently, the ecological significance of the observed ontogenetic shape change is unclear.…”