Ontogenese Du Systeme Nerveux Central Des Chelicerates Et Sa Signification Eco-Ethologique

Summary

Ontogeny of the Central Nervous System in Chelicerates and its Eco-ethological Significance.

The comparative embryogeny of the central nervous system and especially ganglia in annelids, onychophorans, chelicerates, myriapods and crustaceans emphasizes certain characteristics found in all groups and some others limited to a few related groups or even to a single one. A common characteristic of all the groups studied during early development of the nervous system is the occurrence of segmental organs of ectodermal origin which give rise to ganglia.

These organs vary morphologically from one group to another. Several morphological types are distinguishable:

• simple thickenings of the ventral embryonic ectoderm among onychophorans and Symphyla.

• spherical or subspherical organs forming follicles, detached inwards from the ectoderm, in annelids, chilopods, diplopods, xiphosurans, pycnogonids, scorpions, pseudoscorpions, solpugids, spiders and opilions.

• simple ectodermal folds unseparated from the ectoderm and opening outwards in Whip spiders (arachnids, amblypygi).

In different arthropod groups (chelicerates, xiphosurans, pycnogonids, solpugids, gonyleptid opilions), ventral organs seem to be the only ones generating nervous ganglia. In Pachylus quinamavidensis (gonyleptid opilions), six pairs of ventral organs give rise to the cerebral ganglia. This is the highest number found in arthropods; and they may hence be viewed as segmental organs related to neuromeres.

The number of ventral organs in Pachylus is correlated with the development of the neurons of the corpora pedunculata globuli which are integration centres forming 70% of the adult brain. In onychophorans, diplopods, solpugids, opilions and crustaceans, ventral organs persist during adult life.

The primitive embryonic affinities observed during the formation of ganglia in different groups are absent in Insects.

When Hanström published his work on Spiders at the beginning of the century, neuro-ethological studies were difficult to interpret since ecological, ethological and evolutionary data were lacking. Today, the classification of the spiders into four groups by protocerebral structure (Hanström 1935) corresponds to the phylogenetic pattern proposed by Platnick (1971). It would seem that it is the mechano- and chemo-receptive channels of communication of spiders which are correlated with their nocturnal habits. The visual channels and their evolutionary improvement for diurnal life, most elaborate in the Salticidae, appear to be related to the complex protocerebral structure. The change from a nocturnal to a diurnal life is doubtlessly crucial to the question. Hunting spiders (Salticidae) possess the most developed brain. It contains two pairs of optic masses and two integrating centres, the central body and the globuli of the corpora pedunculata. In sedentary spiders, the brain is much less developed; the optic masses are small and less differentiated; the corpora pedunculata are rudimentary, the glomeruli absent while the globuli and peduncle are completely lacking and the bridge rudimentary.

The Lycosid spiders tend towards a diurnal life, but are not as successfully as the Salticids. Neither do their optic structure (Munoz-Cuevas 1984) or their behavior (Tietjen and Rovner 1982) attain the degree of perfection found in the Salticids.

In the neuroanatomical classification by Hanstrom (1935) and that of Platnick (1971) Lycosids are placed at an intermediary level. The structure of the protocerebrum reflects this situation: the optic masses and the corpora pedunculata are less developed in lycosids than in salticids.

This evolutionary process, obvious in spiders, cannot be put forward to explain the highly developed corpora pedunculata in gonyleptid opilions. These Arachnids do not favor visual communication; their activity being, on the contrary, crepuscular or nocturnal.