Background: Many insects jump by storing and releasing energy in elastic structures within their bodies. This allows them to release large amounts of energy in a very short time to jump at very high speeds. The fastest of the insect jumpers, the froghopper, uses a catapult-like elastic mechanism to achieve their jumping prowess in which energy, generated by the slow contraction of muscles, is released suddenly to power rapid and synchronous movements of the hind legs. How is this energy stored?

A comparative ultrastructural study of photoreceptor synapses formed upon homologous postsynaptic neurones in insects has been made by using serial-section electron microscopy in representative Diptera from a monophyletic series of 14 families. At all of the synaptic contacts there is a presynaptic dense bar, surmounted in phylogenetically more recent families by a presynaptic platform. Opposite the bar lies a pair of postsynaptic elements that invariably originate one each from two unique monopolar neurones Li and L2. Both elements contain increasingly elaborate cisternae in more recent flies. Within the phylogenetic series, the postsynaptic ensemble itself changes from the original dyad to a tetradic configuration in more recent Muscomorpha by the addition of two new postsynaptic elements from an amacrine cell. This transition occurs once only in the series, which, gauged by the fossil record, covers divergences from the stem line extending back >200 million years.During the course of evolution, adaptive modifications of the nervous system must have occurred frequently to account for the many corresponding innovations in the behavioral repertoires of animals. How these evolutionary differences between the brains of related species are expressed at the cellular level remains a mystery. Here we distinguish between two contrasting cellular strategies which could alter brain circuitry during the evolutionary divergence ofdifferent phyletic lines: (i) the emergence of new types of neurones, which then become connected so as to modify the circuitry in an existing neuropil, or (ii) alteration in synaptic connectivity among preexisting neurones inherited from a common ancestor. Our comparative study of evolutionary progression in the most peripheral visual neuropil of an insect order, Diptera, supports process (ii) over (i).Since it is difficult to see any obvious homologies between the neurones of different phyla, the generation of new neurone classes is most likely to have been important at the divergence of the major phyla. Within established phyletic lines, however, this mechanism is contraindicated by examples of conservation of neuronal types, particularly clearly in arthropods, in which many of the larger neurones can be recognized uniquely in both sensory and motor pathways.

SUMMARY1. There are problems associated with the notion that slow potentials alone are used to transmit information in the early stages of some visual systems. This idea and alternatives have been tested on the barnacle lateral ocellus, a simple eye with only three photoreceptors, each with its own axon about 1 cm long.2. All of the receptors have very similar properties including spectral sensitivity, and are also electrically coupled together. Impulses cannot be recorded from any of the cell bodies, all ofwhici have been impaled as shown by dye marking.3. No impulses can be recorded externally from most of the ocellar nerve or intracellularly from the receptor axon terminals. Impulses driven by light, sometimes recorded in the final part of the nerve, are believed to originate in other axons.4. During illumination of the eye, current enters the receptor soma and leaves via the rest of the axon. This is consistent with the idea that the axon acts as a purely passive cable. The passive behaviour was also demonstrated in a comparison of the relative attenuation down the axon, of hyperpolarizations and depolarizations.5. Calculations based on the supposed electrical constants of the somas showed that the slow potential itself was unlikely to be the visual signal, since it would be enormously attenuated by passive spread down the long thin axons. To check this, the axon terminals in the supraoesophageal ganglion were penetrated and identified by electrical and dye-marking criteria. In fact, the slow potential was attenuated in the most favourable

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