Some animals have evolved task differentiation of individual structures within the same sensory modality (e.g. olfaction or vision). A particular example is spiders, where most species have eight eyes, of which two (the principle eyes) are used for object discrimination, whereas the other three pairs (secondary eyes) detect movement. In the model spider species Cupiennius salei (Keyserling, 1877) these two eye types correspond to two distinct visual pathways in the brain. Each eye is associated with its own first and second order visual neuropil. The second order neuropils of the principle eyes are connected to the arcuate body, whereas the second order neuropils of the secondary eyes are linked to the mushroom body. However, the secondary eye size and visual fields of C. salei differs considerably from that of the likewise visually hunting jumping spiders. We explored the secondary eye visual pathway of the jumping spider Marpissa muscosa, and found that the connectivity of neuropils differs from that in C. salei. In M. muscosa, all secondary eyes are connected to their own first order visual neuropils. The first order visual neuropils of the anterior lateral and posterior lateral eyes are further connected with two second order visual neuropils, the array of microglomeruli and the 2nd order visual neuropil of the lateral eyes, whereas the posterior median eyes lack associated second order neuropils and axons project only to the arcuate body. These differences between C. salei and M. muscosa suggest substantial differences in information processing and function of their visual neuropils.
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