Strepsipteran brains and effects of miniaturization (Insecta)

Are smaller animals behaviourally limited? Lack of clear constraints in miniature spiders

“…and in adult workers of Pheidole ants, both of which are extremely small insect species (w0.04 mg), the brain is 15e16% of the total body volume or mass (compared to about 2.5% in humans; Polilov & Beutel 2009;Seid et al, in press, unpublished data); this percentage is about 750 times larger than that for the brain of a large water beetle (Beutel et al 2005). In the tiny beetles and in the small first-instar larva of a strepsipteran, the brain extends out of the head capsule and into the prothorax (Beutel et al 2005). The brains of very small orb-weaving spiders are also relatively large, and portions extend into the coxae of the legs and palps (R. Quesada, E. Triana, G. Vargas, M. Seid, J. Douglass, J. Niven, W. Eberhard & W. Wcislo, unpublished data).…”

“…This expectation is based on several considerations. There is a general trend for smaller animals to have larger central nervous systems relative to their body sizes (Beutel et al 2005;Bonner 2006;Wehner et al 2007;Polilov & Beutel 2009;Seid et al, in press); the causes of this trend are not certain (Striedter 2005). Nervous tissue is relatively more costly to maintain than are other types of tissue (e.g.…”

“…The behavioural data needed to answer such a question are very scarce. Beutel et al (2005) and Polilov & Beutel (2009) speculated that there may be no behavioural impairment in the tiny larvae whose morphology they studied, but their arguments cited only the fact that the larvae have sense organs and are able to move about. There are apparently no direct behavioural observations of insects in this size range that might reveal possible manifestations of reduced behavioural abilities, such as increased error rates, slower execution, decreased behavioural repertoires, or other relevant variables.…”

“…Morphological studies show that the brains of very small species of insects and spiders are smaller in absolute terms than those of larger species, as in vertebrates (e.g. Rensch 1960;Bonner 2006), however, they are proportionally much larger (Beutel et al 2005;Wehner et al 2007;Polilov & Beutel 2009;Seid et al, in press). For example, in first-instar larva of the ptiliid beetle Mikado sp.…”

“…The rate of errors in a given task and the speed and agility with which it is performed were not measured. And head size does not necessarily show the same relation to brain size in different species (the brains of smaller species may be larger compared with their head sizes; Beutel et al 2005;Polilov & Beutel 2009;J. Douglass, unpublished data).…”