MECHANISM OF MOUTH PARTS: LOUSE 537 



though, for the same reasons which render the relations of the parts of 

 the proboscis itself so obscure in this region, it is by no means easy to 

 determine their precise attachments. The protractor muscles are of 

 relatively large size ; they arise from the whole extent of the lateral wall 

 of the posterior part of the head (Plate LXVII, fig. 4), behind the 

 brain, the fibres converging in a fan-like manner as they pass around the 

 large mass of nervous tissue which is passing to the neck, and are inserted 

 into the lateral angles of the two chitinous gutters which represent the 

 proboscis in this region. Their general direction is downwards and 

 backwards, and their contraction must therefore have for its result a 

 forward thrust of the proboscis in its sheath. The retractor muscles 

 which oppose this pair are less easy to define. In all sections about the 

 base of the proboscis a considerable number of bundles of muscular and 

 fibrous tissue are to be seen, which do not extend forwards beyond the 

 base of the proboscis, but can be traced backwards to the neck, where 

 they are inclined outwards to the chitinous walls. Some of these prob- 

 ably represent retractors of the labium, others retractors of the maxillae. 

 Without insisting too strongly on the functions of muscles the 

 anatomical relations of which have not yet been determined with 



precision, one may venture to sum up the mechanism 



. i " j i r 11 Mechanism of the 



of the proboscis and sucking apparatus as follows. 



/T . , , mouth parts and 



The anterior end of the head is thrust against the skin suckjn g appara t u8 



of the host with the teeth in their normal closed 

 position, the hind end of the body being at the same time elevated in 

 order to assist the insect in gaining a purchase, and to bring the mouth 

 parts into a suitable position. The protractor muscles of the buccal 

 cavity then contract, and as the organ is thrust bodily forwards the teeth 

 are rotated on their bases so that their points are turned outwards and 

 thrust into the skin. Possibly this action is repeated several times until 

 the outer cuticle is pierced and the head firmly attached to the skin. 

 According to many observers, among them Pawlowsky, the most recent 

 writer on the subject, the louse chooses the opening of a sweat gland for 

 its point of puncture.* The next occurrence is the protraction of the 

 proboscis from its sheath into the wound. It emerges at the mouth 

 orifice between the two rows of teeth which make up the circlet, and is 

 alternately protracted and retracted until the teeth on the maxillae have 

 bored a hole down to the level of blood. The maxillae then form the 

 channel up which the blood flows to the buccal cavity, and at the same 



* The authors have never been able to satisfy themselves, even after repeated observa- 

 tions, that this is actually the case. 



68 



