Adaptations for Nocturnal Vision in Insect Apposition Eyes

Due to our own preference for bright light, we tend to forget that many insects are active in very dim light. Nocturnal insects possess in general superposition compound eyes. This eye design is truly optimized for dim light as photons can be gathered through large apertures comprised of hundreds of lenses. In apposition eyes, on the other hand, the aperture consists of a single lens resulting in a poor photon catch and unreliable vision in dim light. Apposition eyes are therefore typically found in day-active insects. Some nocturnal insects have nevertheless managed the transition to a strictly nocturnal lifestyle while retaining their highly unsuitable apposition eye design. Large lenses and wide photoreceptors enhance the sensitivity of nocturnal apposition eyes. However, as the gain of these optical adaptations is limited and not sufficient for vision in dim light, additional neural adaptations in the form of spatial and temporal summation are necessary.

The second and third optic ganglia of the worker bee: Golgi studies of the neuronal elements in the medulla and lobula

The gross morphology and the fine-structural characteristics of neurones of the second and third optic ganglia of the honeybee Apis mellifera were investigated light microscopically on the basis of Golgi (selective silver)- and reduced silver preparations. The second optic ganglion, the medulla, is ovoid in shape and has a slightly convex distal surface and a slightly concave proximal surface. The medullar outer levels are characteristically composed of neuronal arrangements showing strict precision of their geometrical spacing proximally as far as a pronounced layer of tangential fibre elements comprising the serpentine layer of the medulla. At the inner medullary levels retinotopic channels are again multiplied, and the arrangement of axons and dendrites contribute to a complex lattice. The third optic ganglion, the lobula, is interposed between the medulla and the protocerebrum. It is the site of termination of the third-order neurones. The lobula in hymenopterans appears, in contrast to dipterans, odonates and lepidopterans, as a single neuropilic mass. A short review of the electrophysiological data concerning these two ganglia has been tentatively correlated with some of the anatomical data.

Types and arrangements of neurons in the crayfish optic lamina

The neural arrangements in the optic lamina of the crayfish Pacifastacus leniusculus Dana have been studied by light microscopy by means of silver impregnation techniques. The lamina is composed of columnar synaptic compartments (cartridges). Each cartridge is composed of seven receptor terminals distributed in two layers and second-order monopolar neurons connecting the lamina with the second synaptic region, the medulla externa. The neurons found in the lamina consist of five classes: monopolar neurons, centrifugal small-field neurons, tangential neurons, multipolar cells (possibly of a glial nature) and photoreceptor axons (fig. 13). Among the monopolar cells, five types are classified (M1-M5) according to their lamina arborizations. Two types are stratified (M3 and M5) corresponding to the photoreceptor terminal strata. On this basis, the lamina plexiform layer is subdivided into two layers (epl1 and epl2). The remaining monopolar neurons have lateral processes in both layers, two of them within one cartridge (M1 and M2) and one over several cartridges (M5). There is one type of small-field centrifugal neuron (C1) and two types of tangential medulla to lamina neurons (Tan1 and Tan2), both having processes covering a large number of cartridges. Multipolar cells with cell bodies distal (MP1) or proximal (MP2) to the plexiform layer send processes to several cartridges. The receptor axons consit of three types. One has terminals in epl1 or epl2, the second has its terminal in epl1 and a thin process to epl2, and the third (corresponding to the 8th retinular cell) bypasses the lamina and has a terminal in the medulla externa. A brief comparison is made with the neural arrangements in the lamina of the Norway lobster Nephrops norvegicus L.

Fine structure of the first optic ganglion (lamina) of the cockroach, Periplaneta americana

The stuctural organization of the first optic ganglion (lamina) of the cockroach (Periplaneta americana) was investigated by the use of light and electron microscopy. Each compound eye of the cockroach is composed of up to 2000 visual units (ommatidia) of the fused rhabdom type. The ommatidia themselves consist of eight receptor cells which terminate as axons in either the first or second optic ganglion. Three different short visual fibre types end in two separate strata in the lamina, and one long fibre type ends in the second optic ganglion. Monopolar second-order neurons with wide field branching patterns in the middle stratum of the first synaptic region have postsynaptic contacts with sort visual fibres. Horizontal fibre elements with branching patterns at different levels of the lamina apparently from three horizontal plexuses with presynaptic and/or postsynaptic connections to first-and second-order neurons. The lack of well-organized fibre cartridges containing a constant number of first and second order neurons in each fascicle and the presence of only unistratified wide field monopolar cells could represent, as compared to other insect orders, a primitive stage in the development of the first optic ganglion.