124 THE STRUCTURE OF THE nEAD OF THE BLOWFLY LARVA. 



M. Menzbier* thinks the bead is developed from six pairs of histo- 

 blasts, but as be confesses be cannot find them, this may be regarded 

 as a mere opinion. It cannot be said that the further development of 

 the histoblasts of the galea has been traced, but these are the largest 

 in the head if we omit the procephalic discs. Any others which 

 may exist must be of small size. The conclusion at which I have 

 arrived is that these form the greater part of the proboscis. 



If the above view proves correct, the homologies of the proboscis 

 and its relations to the larval and embryo states of development 

 becomes simple. The whole exterior of the proboscis, except the 

 lab rum, would represent the galeae and stipes of the maxillae, whilst 

 the edges of the labrum and its apodemes represent the lacinia or 

 mando. Under this view the position of the maxillary palpi is no 

 longer abnormal. 



With regard to the labium the coalescence of the labial histo- 

 blasts with the salivary duct indicates that it forms the floor of 

 the groove between the lips and the basal joint of the proboscis, 

 that is, the labium is retracted within the galeae of the maxillae, and 

 these have coalesced behind. 



The intimate relation of the salivary duct with the labium of the 

 larva, and the existence of a suctorial or quasi-suctorial disc around 

 the mouth of the larva with rudimentary pseudotracheal grooves, are 

 very suggestive. The existence of a large histoblast or imaginal 

 disc in this region is not to be overlooked, and I would add the 

 pharynx of the larva is almost identical with that of the imago. 

 All these points are distinctly in favour of the view I have endea- 

 voured to place before you. 



It may appear to some that so remarkable a modification of the 

 maxillaef is improbable, but on this point I will merely observe now 

 that the maxillae exhibit a wide deviation from their primitive form 

 in the Lepidoptera, and that the maxilla being serially homolo- 

 gous with the labium, there is no a priori ground for denying simi- 

 lar deviations from their primitive form in the two cases. If the 

 paraglossae of the labium are capable of so wide a modification, 

 why not the galea of the maxilla ? Hereafter I shall show that the 

 structure of the mouth-parts of other diptera is consistent with the 

 view 1 have laid before you. 



* Loc cit. 



t Brulle — " Researches sur les transformations des appendices dans les 

 articules " (" Ann. Sc. Nat. ' 3rd ser., torn, ii.) — speaks of the proboscis as 

 consisting largely of the galea ; either we must admit that the maxillae are 

 united with each other or with all the other parts of the mouth. 



