1916.] 189 



The morphology of the larva is described in detail. There are considerable 

 differences between the first and the two succeeding- stadia of the larva. The 

 first-stage larva is furnished on the ventral side of the abdomen with hooks 

 which are absent in the later stages. The bucco-pharyngeal armature also is 

 peculiar in the first stadium, inter alia having in front a single median mouth- 

 hook instead of the paired hooks of the later stages. 



The primaiy larvae of the Muscid Onesia sepulchralis have also been found 

 in certain species of Allolobophora, as have also certain later-stage larvae 

 probably belonging to that species. It seems possible that this insect may 

 have a liCe-cycle not unlike that of Pollenia rudis. The larvae of Onesia cognata 

 have once been found living in sores on the bodies of nestling wagtails (ilfo- 

 tacilla alha (p. 95 sqq.)) : but Keilin suggests that this may be quite exceptional. 



The conclusions of the second part of the treatise, on the Comparative 

 Biology and Morphology of Dipterous larvae, can only be very briefly 

 summarised, (a) The phases in the life-cycle of Pollenia rudis correspond to 

 those in the cycles of Muscoid flies parasitic on Arthropods : for these latter 

 also show (i) a phase of non-fixation, in the body-cavity of the host ; (ii) phase 

 of migration ; (iii) perforation ; (iv) phase of fixation, in which the parasite is 

 siiccessively plasmophagoiis, sarcophagous, saprophagous ; (v) emergence from 

 remains of host and free life ; (vi) pupation. The life-cycles of some Dipterous 

 parasites of Vertebrates, such as Hypoderma, also exhibit phases comparable 

 with those of Pollenia, especially in the inflammation of the sun-ounding host- 

 tissues during the fixed phase. Other parasites of Vertebrates, such as Sarco- 

 phaga, Cordylobia, and Oestrus have a much more simple and direct cycle. 

 Some striking peculiarities in the life-history of Pollenia are (i) the phagocytic 

 reaction of the host ; (ii) the possibility of expulsion from the host; (iii) the 

 fact that when the pax'asite comniences to feed, the host ceases to do so. (b) the 

 essential difference in the behaviour of Dipterous and Hymenopterous parasites 

 lies in this, that the former must put their spiracles in communication with 

 the outer air, whence largely arises the complication of their life-histories. 

 This necessity affects all Dipterous larvae, free-living as well as parasitic, 

 aquatic as well as terrestrial. (c) The bucco-pharyngeal armatitre shows 

 certain constant differences in detail which characterise the great ethologic 

 groups (parasitic, carnivorous, predaceous, saprophagous, and phytophagous 

 forms, and species passing the whole larval life in the body of the parent) 

 irrespective of systematic relationship, (d) All Cyclorhaphovxs larvae pass 

 throiigh three stages separated by two moults. Pollenia is typical in this 

 respect, that the great morphological divergence is between the first and the 

 two siicceeding stadia, not between the two latter, which closely resemble one 

 another, (b) The greatest differences between Orthorhaphous and Cyclor- 

 haphous Dxptera lie in the forms of the larvae, the morphology of which is 

 entirely unlike. For variovis reasons Keilin suggests that all Cyclorhaphous 

 larvae have had a parasitic origin, the free-living kinds being only secondarily 

 free-living, (f) All Dipterous larvae possess certain minute sensory organs 

 in the places where the thoracic legs would be were they present ; these organs 

 are to be regarded as sensory vestiges of the legs. 



