32 BRITISH FLIES 



larvae can usually move freely by means of projections of the body called pseudopods, 

 or by the aid of short bristles arranged so as to favour progression even in completely 

 maggot-like forms. The spiracles (to which we shall subsequently allude) are also 

 of great aid in enabling us to decide whether a larva belongs to Diptera or not. 

 The more important of the characters available for the discrimination of the kinds 

 of Dipterous larvae are ; ] , the general form or shape, which is extremely varied ; 

 2, the nature of the head : 3, the spiracles. 



1. Under the heading of general form may be included the number of segments 

 behind the head. This is said by 8cliiner to be twelve, three of which are called 

 thoracic segments, and the others abdominal. Frequently the number apparent is 

 less than twelve. There is, moreover, no morphological criterion yet discovered by 

 which the segments can be numl)ered, and in many cases the segments cannot be 

 satisfactorily delimited in the present state of knowledge. Considerable work at the 

 cmbryological development is necessary for the satisfactory elucidation of this 

 elementary point. This difficulty is greatly increased by the remarkable variety as 

 regards the head, to Avhich we shall presently refer. In Brauer's larger table it will 

 be noticed that in certain cases he uses a qualified expression instead of segment. 

 In hJepharoceriche (not yet discovered in Britain) there appear to be only five or 

 six segments in the larva, while in Thereva there are some nineteen or twenty. In 

 the latter case the term " Zwischen-segment '' is used by Brauer ; the intercalated 

 segments are supposed to be due to the connecting membranes between certain 

 segments taking on the appearance of true segments. 



2. The head. — One of the characteristics of Diptera is the frequent occurrence of 

 diminution in size of the head, a character Avhich we find repeated in parasitic 

 Hymenoptera and to a considerable extent in the social Hymenoptera where the 

 larvae are fed by the parents. The number of Dipterous families in which there is 

 a large head is but small. The most conspicuous case is the mosquitoes or Culicichi^. 

 This condition of the head is called " eucephalous.'"' At the opposite extreme we find 

 an apparently complete absence of a head, as in the fiesh-feeding maggots. This is 

 the acephalous condition. There is no means whatever of framing an anatomical 

 definition of Eucephala and Acephala ; indeed the majority of the families of 

 Dipterous larva? are in an intermediate state. The term " hemicephalous " larvaj 

 has been used by Dvifour and others ; it may be applied to those forms in which the 

 skin of the neck is attached, not to the base of the head, but to the middle of it 

 as in Tvpidicki:' and Stratiomj/icht, cf. our fig. of Stratiomys (fig. 54), so that the back 

 part of the head is permanently inside the body. In many such cases the front part 

 of the head can be completely drawn back within the following segments, so that the 

 larva is then to all appearance acephalous. The student must be on his guard as to 

 this point. After the separation of these more or less definite forms of head, there 

 ronain a very long series of cases (including most of the families dealt with in this 

 volume) in which the head is reduced to a greater or less extent in size, and in some 

 becomes of peculiar form. In some the head is always held exserted • these are the 

 forms that approximate to the eucephalous condition. In other cases the larva usually 

 draws its head inside its body, exserting it only for purposes of movement or feeding. 

 These forms {Tahanidw, etc.) approximate to the acephalous condition. 



3. The spiracles. — Dipterous larvae have the last pair of spiracles largely developed, 

 and they are frequently placed at the actual tip of the body ; when not at the tip 

 they are usually placed dorsally rather than laterally. These are points of distinction 

 as compared with other orders of insects. Moreover when the spiracles are placed 

 at the tip of the body there frequently exist special arrangements for protecting 

 them ; and in these cases, to a superficial examination, these spiracles may appear 

 to be quite absent. In TipuluJo' this is the case Ijecause the extremity of the body 

 consists of soft processes that fold and contract, and so conceal the breathing organs ; 

 but every now and then the processes are expanded and the spiracles become 

 very conspicuous. In the StnitiomyUke the posterior sj^iracles are placed in a 

 special respiratory chamber at the tip of the l)ody, and are therefore difficult of 

 detection. 



When the posterior spiracles are the only pair that exists the larva is said to lie 

 metapneustic ; when in addition to these there is an anterior pair jilaced a littU' 

 behind the head the larva is amphipneustic ; wiien there are also intermediate spiracles 

 the larva is peripneustic. The freiiuency of the metapneustic and amphipneustic 

 systems is characteristic of Di^jtera ; the peripneustic system being the usual one in 

 other orders. 



