120 FLEAS, FLUKES AND CUCKOOS 



The strongly-toothed, dark-coloured mandibles (Plate XXI) can be 

 seen either near the centre of the head (Ischnocera*, Plate XXII) or 

 near the anterior margin (Amblycera* Plate XXII). These are used 

 to cut off pieces of feather, usually of constant lengths, which fall into 

 the pouch-like labrum or forelip. Movements of this pouch force the 

 food into the mouth. The maxillae and labium are reduced to simple 

 lobes either without palps (Ischnocera) or with segmented maxillary 

 palps (Amblycera), and probably play only a minor part in the feeding 

 operation. 



The feather-feeding forms generally take the down or downy 

 part of the larger feathers. When feeding the louse approaches the barb 

 of the feather head foremost and hangs to it by the second and third pair 

 of legs, the first pair being used to direct a single feather barbule 

 towards the mandibles. The pieces of feather, cut by the mandibles 

 and forced into the mouth by the labrum, pass down the oesophagus to 

 the crop. When full this shows as a black structure lying in the abdo- 

 men of the living insect, rounded anteriorly and pointed posteriorly 

 (Plate la). If a louse is watched while feeding, strong pulsating move- 

 ments of the crop can be seen. These movements help in the breaking 

 up of the feather parts, particularly by rubbing them against sets of 

 comb-like structures in the wall of the fore-part of the crop. Small 

 mineral granules are sometimes found in the crop, and it has been 

 suggested that these may act as further grinding agents — an interesting 

 analogy to the grit in the gizzard of the bird. But more than purely 

 mechanical treatment is needed for the digestion of the food. Feathers 

 consist mainly of keratin — a strongly resistant substance — and before 

 this can be acted on by the digestive enzymes of the gut it must be 

 subjected to a strong reducing agent. Such an agent is secreted in the 

 stomach of the louse; the larva of the clothes moth, which also feeds 

 mainly on keratin, secretes a similar substance. The protein-digesting 

 enzyme of the louse is also adapted to enable it to digest the specialised 

 protein of the keratin, when the latter has been broken down by the 

 first secretion. 



Some of the feather-lice harbour bacteria, which are confined to 

 special cells lying in groups in various parts of the body. Elaborate 

 methods have been evolved by which the bacteria infect the breeding 

 organs of the female and are passed into the egg^ thus ensuring the 

 continued association between bacteria and louse. The exact role 

 ♦See p. 139 for explanation of these superfamilies. 



