The structure of the sensilium of fleas has been investigated by light, stereoscan and transmission electron microscopy.
This compound sensory structure, which occurs in both sexes, is positioned on the terminal segments and superficially comprises a saddle-shaped cuticular plate covered with microtrichia between which are dispersed a species-specific number of pits (16–100) each with a long hair arising from its centre. The cuticular pit is made of two chambers each supported by vertical septa. The apertures in each chamber through which the seta passes vary in diameter allowing an angle of flexure of the hair.
Observations of cellular detail indicate features common with other insect mechano-receptors, a bipolar neuron making contact with the hair base. The dendrite is divided into distal and proximal segments by a ciliary region in which can be seen supporting scolopale rods. The scolopale cell, the trichogen and tormogen cells ensheath the sensory neuron. The tormogen cell is large and one is characteristically associated with each pit. During development it withdraws from the hair leaving a large extracellular cavity into which protrude numerous microvilli.
In Ceretophyllus styx secretion is found in this extracellular cavity during the Spring period prior to the return of the migrant host.
The tormogen cell wraps around the neuron and is thought to give support to the hair base/neuron (= tubular body) junction giving a measure of increased sensitivity to the system. This sensitivity, it is suggested, could be increased by the presence of the extracellular secretion which is serving to hold the system rigid, any movement of the hair being passed on to the neuron.
C. styx overwinters in a cocoon emerging in the Spring when its migrant host returns. On emergence the system, sensitised by the presence of the secretion, can now be used for the detection of the returning host by mechanical means and/or during the mating process, aligning male with female. Ceratophyllid fleas are known to mate on emergence without the necessity of a blood meal.
Having mated removal of the fluid would make the hair base/neuron junction less sensitive.
If the interpretation of the results is accepted and the presence of fluid in C. styx at least, is a physiological feature at certain times of the year, then two possible roles exist:
(1) That the sensilium is used to detect the presence of the returning host.
(2) That, having emerged from the overwintering cocoon, the sensilium is then used in the mating process.