http://chittkalab.sbcs.qmul.ac.uk/2009/ChittkaNivenCurrBiol09.pdf

Are Bigger Brains Better

The eusocial bumblebees exhibit pronounced size variation among workers of the same colony. Differently sized workers engage in different tasks (alloethism); large individuals are found to have a higher probability to leave the colony and search for food, whereas small workers tend to stay inside the nest and attend to nest duties. We investigated the effect of size variation on morphology and physiology of the peripheral olfactory system and the behavioral response thresholds to odors in workers of Bombus terrestris. Number and density of olfactory sensilla on the antennae correlate significantly with worker size. Consistent with these morphological changes, we found that antennal sensitivity to odors increases with body size. Antennae of large individuals show higher electroantennogram responses to a given odor concentration than those of smaller nestmates. This finding indicates that large antennae exhibit an increased capability to catch odor molecules and thus are more sensitive to odors than small antennae. We confirmed this prediction in a dual choice behavioral experiment showing that large workers indeed are able to respond correctly to much lower odor concentrations than small workers. Learning performance in these experiments did not differ between small and large bumblebees. Our results clearly show that, in the social bumblebees, variation in olfactory sensilla number due to size differences among workers strongly affects individual odor sensitivity. We speculate that superior odor sensitivity of large workers has favored size-related division of labor in bumblebee colonies.

Size determines antennal sensitivity and behavioral threshold to odors in bumblebee workers

Miniaturization leads to considerable reorganization of structures in insects, affecting almost all organs and tissues. In the smallest insects, comparable in size to unicellular organisms, modifications arise not only at the level of organs, but also at the cellular level. Miniaturization is accompanied by allometric changes in many organ systems. The consequences of miniaturization displayed by different insect taxa include both common and unique changes. Because the smallest insects are among the smallest metazoans and have the most complex organization among organisms of the same size, their peculiar structural features and the factors that limit their miniaturization are of considerable theoretical interest to general biology.

Small Is Beautiful: Features of the Smallest Insects and Limits to Miniaturization

Animals use vision to perform such diverse behaviors as finding food, interacting socially with other animals, choosing a mate, and avoiding predators These behaviors are complex and the visual system must process color, motion, and pattern cues efficiently so that animals can respond to relevant stimuli The visual system achieves this by dividing visual information into separate pathways, but to what extent are these parallel streams separated in the brain? To answer this question, we recorded intracellularly in vivo from 105 morphologically identified neurons in the lobula, a major visual processing structure of bumblebees (Bombus impatiens) We found that these cells have anatomically segregated dendritic inputs confined to one or two of six lobula layers Lobula neurons exhibit physiological characteristics common to their respective input layer Cells with arborizations in layers 1–4 are generally indifferent to color but sensitive to motion, whereas layer 5–6 neurons often respond to both color and motion cues Furthermore, the temporal characteristics of these responses differ systematically with dendritic branching pattern Some layers are more temporally precise, whereas others are less precise but more reliable across trials Because different layers send projections to different regions of the central brain, we hypothesize that the anatomical layers of the lobula are the structural basis for the segregation of visual information into color, motion, and stimulus timing

/pdf/the-processing-of-color-motion-and-stimulus-timing-are-ka48xewuuu.pdf

The Processing of Color, Motion, and Stimulus Timing Are Anatomically Segregated in the Bumblebee Brain

Neural processing in the brain controls behavior through descending neurons (DNs) - neurons which carry signals from the brain to the spinal cord (or thoracic ganglia in insects). Because DNs arise from multiple circuits in the brain, the numerical simplicity and availability of genetic tools make Drosophila a tractable model for understanding descending motor control. As a first step towards a comprehensive study of descending motor control, here we estimate the number and distribution of DNs in the Drosophila brain. We labeled DNs by backfilling them with dextran dye applied to the neck connective and estimated that there are ~1100 DNs distributed in 6 clusters in Drosophila. To assess the distribution of DNs by neurotransmitters, we labeled DNs in flies in which neurons expressing the major neurotransmitters were also labeled. We found DNs belonging to every neurotransmitter class we tested: acetylcholine, GABA, glutamate, serotonin, dopamine and octopamine. Both the major excitatory neurotransmitter (acetylcholine) and the major inhibitory neurotransmitter (GABA) are employed equally; this stands in contrast to vertebrate DNs which are predominantly excitatory. By comparing the distribution of DNs in Drosophila to those reported previously in other insects, we conclude that the organization of DNs in insects is highly conserved.

/pdf/organization-of-descending-neurons-in-drosophila-140me6o6up.pdf

Organization of descending neurons in Drosophila melanogaster.

Within a particular animal taxon, larger bodied species generally have larger brains. Increased brain size usually correlates with increased behavioral repertoires and often with superior cognitive abilities. Bumblebees are eusocial insects that show pronounced size polymorphism among workers, whereas in honey bees size variation is much less pronounced. Recent studies suggest that within a given colony, large bumblebee workers are more efficient foragers and are better learners than their smaller sisters. Here we examine the allometric relationship between brain and body size of worker bumblebees and honey bees. We find that larger bees have larger brains and that most brain components show a similar size increase as the overall brain. One particular brain structure, the central body, is relatively smaller in large bumblebees compared to small bees. The same is true for the mushroom body lobes, whereas the mushroom body calyces, which receive sensory input, are not reduced in larger bumblebees or honey bees. Honey bees have relatively smaller brains, as well as smaller mushroom bodies, than bumblebee workers. We discuss why brain or mushroom body size does not necessarily correlate with the degree of a species' social organization.

/pdf/brain-allometry-in-bumblebee-and-honey-bee-workers-380juqlkoo.pdf

Brain Allometry in Bumblebee and Honey Bee Workers

Unique among insects, some paper wasp species recognize conspecific facial patterns during social communication. To evaluate whether specialized brain structures are involved in this task, we measured

Correlation between facial pattern recognition and brain composition in paper wasps.

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Brain Allometry in Bumblebee and Honey Bee Workers

Correlation between facial pattern recognition and brain composition in paper wasps.