Onchidium (Gastropod, Mollusca) has a multiple photoreceptive system (MPS) located in the different regions of the body, 1) a pair of stalk eye on the tip of tentacles, 2) dorsal eyes on the dorsal mantle surface, 3) free dermal photoreceptor cells (DPC) in the dermis, and 4) some photosensitive neurons in the central nervous system. Two different types of photoreceptor cells, ciliary-type photoreceptor cell (CC) and microvillous-type lens cell (LC) are presented in the same dorsal eye. The photoreceptor cell (VC) in the stalk eye retina, and DPC are microvillous-type, and characterized with well-developed microvilli (MV) and abundant photic vesicles (PVs) in the cytoplasm. Both organelles are typical of photoreceptor cell in gastropod eyes. The LC has well-developed MV as in DPC and VC, but the cytoplasm are occupied with endoplasmic reticulum (ER) instead of PVs. The MV of three microvillous-type cells in the Onchidium MPS were positively stained with anti-squid rhodopsin antiserum, and PVs and ER showed positive staining with anti-retinochrome peptide antibody. It suggested the existence of the rhodopsin-retinochrome system. The CC in the dorsal eye retina showed hyperpolarized response for light, but three microvillous-type photoreceptor cells showed depolarized response. The dorsal eye appears to be maxillary sensitive to blue light, while VC in the stalk eye and DPC are maximally sensitive for green light.

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The ability to learn to associate a certain cue with an abundant food source must be of great significance for insects living in constantly changing environments, where the availability of food sources varies with the seasons. I found that the cricket could associate an odor to water reward. Here I review recent progress on the olfactory learning capabilities of the crickets, which provide solid basis for future studies of neural mechanisms of olfactory learning and memory. I examined the odor learning ability of cricket in terms of the length of memory retention, memory storage capacity and context learning. Behavioral experiments clarified the followings: 1) Olfactory memory established in nymph was kept for life, but it could be re-written easily. 2) Crickets could associate seven pairs of odors to water reward simultaneously. 3) Crickets have a capability to select one of a pair of odors and to avoid the other in one context and to do the opposite in another context. In various animals, it has been demonstrated that memory can be divided into several distinct temporal phases, e.g. short-term memory and long-term memory. Behavioral and pharmacological experiments in crickets demonstrated that memory after olfactory conditioning could be divided into 4 different phases.

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