Nothing is known about the flea’s powers of vision, but there is no reason to suppose that they are at all acute. The eyes are marked with pigment. Ocelli appear to be primitive types of insect eye which are, perhaps, an inheritance from a wormlike ancestor. Presumably all the fleas of long ago had eyes and those that are now blind have lost their organs of sight from disuse. In their simplest condition, the eyes of the lower invertebrates only enable the creature, so far as one can judge, to distinguish light from darkness. Entomologists believe that the power of vision of ocelli is probably confined to very near objects and that this simple form of eye is more useful in dark places than the compound eyes. There is no reason for believing that fleas can distinguish colours or can discern any object which is more than a few inches away. It is enough for their purpose to perceive from which point light comes upon them and to make all despatch to escape in the opposite direction.
In blind fleas there is often a spine where the eye should be. In one species the spine is rudimentary and there is some black pigment beneath it. It is not impossible that this is the vestige of a once functional eye. In one genus, however, the eye and the spine are both present. Of the fleas belonging to this genus one species is South American and the other European. The latter (Typhloceras poppei) is confined to the long-tailed field-mouse.
The organs by which fleas keep in touch with the outward world, and with other fleas, are their antennæ. All fleas have antennæ; but unlike those of a moth, a beetle, or a grasshopper, each fits neatly into a groove at the side of the head and can be protruded when desired. This is another adaptation to enable the insect to creep swiftly through a forest of hairs.
Fig. 2. Showing a type of (a) genal and (b) thoracic combs of a flea, on the under part of the head and on the thorax respectively. Analagous combs are found in several other parasitic insects and on the abdominal segments of certain fleas.
The combs which are found on the heads of many fleas are organs of exceptional interest ([Fig. 2]). They are toothed and horny appendages, which are connected with parasitic habits, for somewhat similar combs are found on several unrelated groups of parasitic insects, as, for instance, on parasitic beetles (Platypsyllus) found on the beaver, on insects allied to bed-bugs (Polyctenes) found on bats, and on wingless flies (Nycteribia) which infest Egyptian and South American bats. The majority of the Pulicidæ have one or more combs with comparatively long teeth. These combs reach their maximum development in the bat-fleas which have no less than eight. Some Australian and South American fleas (Stephanocircus) have a helmet-like comb extending all round their heads. These combs are by some supposed to be of service in holding on to the hairs of the host; and, if one may judge from experiments made on live fleas in cotton wool, they are also used in moving forwards through the fur.
All the chigoes (Sarcopsyllidæ) have a large triangular post-oral process which is more or less curved and probably prevents the flea slipping back as it pushes forward. Bat-fleas (Ceratopsyllidæ) have lobes or flaps placed two on each side of the head, which may possibly serve an identical purpose, but whether they do so is not known.
These combs may be divided into three groups according to the part of the flea’s body on which they are found. Those found on the head are called genal combs and take the form shown in [Fig. 2]. There are also combs found on the thorax. The fossil flea (Palæopsylla klebsi) described in the previous chapter has one of these thoracic combs. A certain number of fleas also have combs on the abdominal segments.
There are really two types of toothed organs on fleas to both of which the name of comb may be given. One is composed of a sheet of chitin with a number of slits and teeth and resembles a true comb. The other consists merely of a number of highly chitinised bristles arranged in a row. They probably both serve the same function. Apart from their use as organs to assist movement onwards, they may also serve as hair-tight joints and protect the flea from the inconvenience of getting the tips of the host’s fur into the joints of its horny armour.