464 
Head of Psylla mali 
deep parallel-walled channel, which opens out on the front face of the labium. 
Within the terminal joint, this channel becomes relatively shallower following 
upon the reduction of the diameter of the segment, and at the tip the groove 
containing the setae, in addition to becoming contracted, is considerably 
strengthened by reason of the added thickness of its walls. As a consequence 
of this, the setae are firmly enclosed so that in some preparations in which, 
from the treatment to which they have been subjected, the upper parts of the 
setae have become displaced from the groove, the terminal portions will still 
remain securely retained in position within the tip of the proboscis. 
The Setae. 
The setae of P. mali are of the usual Homopterous type. They are four in 
number, arranged in pairs, one pair being anterior to the other. As has been 
indicated in the first part of this paper, the homologies of the Hemipterous 
setae are now generally accepted, so that it may be taken that the anterior 
pair are the mandibles, and the posterior pair the maxillary setae. Each seta 
consists of a hollow, conical, expanded basal portion within which is to be 
found a glandular or deeply staining body which corresponds to the “retorten- 
formigen Organe” of the German writers; and a long slender portion 
terminating in a point which constitutes the piercing stylet. The basal 
portions of the &etae are contained within the head, but the long tapering 
stylets pass out through the buccal cavity, and eventually become lodged in a 
groove on the anterior face of the proboscis, which they traverse to its tip. 
The mandibular setae are free throughout their entire length, are somewhat 
stouter in build than the maxillary setae, and the cavity of the base of the 
seta is continued into the stylar portion in the form of a fine tube which 
traverses the stylet throughout its whole length. This tube, however, does not 
communicate with the exterior, but becomes closed when the stylet com¬ 
mences to taper off to form the pointed tip. In cross section, the mandibular 
stylet takes the form of an ellipse, one side of which, viz. the face which is 
applied to the maxillary seta of its side, is flattened (text-fig. 1). 
The maxillary setae are rather more slender in form and, as they pass out 
from the head into the buccal cavity, each approaches the median line until, 
eventually, they become closely applied one to the other, so as to form a single 
piercing organ. At first this association consists merely of juxtaposition, but 
soon after the setae have left the buccal cavity they become more closely 
united. This is effected by the presence in each seta of a narrow groove on its 
inner face, which receives a corresponding ridge on the other seta so that the 
two parts become firmly fixed together. This interlocking becomes pro¬ 
gressively more complete the nearer the tip of the organ is approached, but 
as far as can be determined, the two parts do not actually fuse together, though 
at the distal end it is apparently difficult to separate them. In the more 
proximal portions, however, the interconnection between the parts is not 
nearly so complete so that the outline of each component part can be clearly 
