Fine structure of the tibiotarsal and pretarsal sensory organs in Monobella grassei banyulensis Deharveng (Collembola : Neanuridae)

Abstract

The fine structure of the tibiotarsal and pretarsal sensory organs of Monobella grassei banyulensis Deharveng (Collembola : Neanuridae) has been examined by electron microscopy. Three types of sensory organs have been observed. (1) the most numerous setae of the tibiotarsus are classic mechanosensitive setae with one bipolar sensory cell, whose distal outer segment ends in a tubular body. (2) Two small setae are arranged on each side of the basal part of the claw; they show 3 sensory cells, 2 of which are mechanosensitive cells of the scolopidial type; the outer segments of the 2 mechanosensitive cells end at the base of the sensory hair. The dendrite of the 3rd sensory cell extends into the hair shaft. (3) Two similar chordotonal sensilla link the tibiotarsus and the pretarsus; each sensillum is composed of 2 bipolar sensory cells enveloped in sheath cells. The first type of sensory organ shows the characteristics of insect exteroceptive mechanosensitive hairs. The mechanosensitive cells of the 2nd and 3rd tibiotarsus sensory organs are probably proprioceptive and control the movements of the pretarsus in relation to the tibiotarsus. Two features are noteworthy: (1) the association of the scolopidial cells with a chemosensitive one has never been observed in other insect sensory organs, except in the Collembola; and (2) there is an important morphological diversity in the ciliary roots of the various scolopidial cells, which are in other respects very similar.