ITS OUTER SKELETON. 55 



are turned up, and the siphon becomes dorsal. The more 

 specialised flies have the simple arrangement of the Bug com- 

 plicated by a s\ T stem of branching tubes, which are probably a 

 special modification of the salivary duct. Similar as the 

 mouth-parts of the four types may be in regard to their mode 

 of working, they cannot be reduced to any common plan which 

 differs materially from that presented by the jaws of the 

 Cockroach. 



Composition of Head. 



In all Insects fusion of the primitive elements of the head 

 begins so early and is carried so far, that it is extremely difficult 

 to discover the precise way in which they are fitted together. 

 The following facts have been ascertained respecting the develop- 

 ment of the parts in question. At a very early stage of 

 embryonic life the body of the Insect becomes divided into a 

 series of segments, which are at fewest fourteen (in some 

 Diptera), while they are not known to exceed seventeen.* Each 

 segment is normally provided with a pair of appendages. The 

 foremost segment soon enlarges beyond the rest, and becomes 

 divided by a median groove into two " procephalic lobes. "f Of 

 the appendages the first eight pairs are usually more prominent 

 than the rest, and of different form ; those of the eighth 

 segment, which may be altogether inconspicuous, never attain 

 any functional importance. The first four pairs of appendages 

 are budded oif from the future head, while the next three pairs 

 form the walking legs, and are carried upon the thoracic segments. 

 All the existing appendages of the fore part of the body are 

 thus accounted for, but the exact mode of formation of the head 

 has not yet been made out. The chief part of its walls, includ- 

 ing the clypeus, the compound eyes, and the epicranial plates, 

 arise from the procephalic lobes, and represent the much altered 

 segment of which the antennae are the appendages. The labrum 

 is a secondary outgrowth from this segment, and, in some cases 

 at least, it originates as a pair of processes w r hich resemble true 



* Balfour, Embryology, Vol. I., p. 337. 



t Huxley, Med. Times and Gazette, 1856-7; Linn. Trans., Vol. XXII. , p. 221, 

 and pi. 38 (1858). 



