WERNER MARCHAND 143 



The finer structure of the chitinous layer is easily studied on cross 

 sections of the larva which is fixed and stained before. On section 

 the chitinous layer is found to consist of three zones; first, an inner 

 zone, which is in immediate contact with the hypothelium; second, a 

 median zone, resting on the former; third, an external zone, which 

 limits the body of the larva exteriorly. These three zones present 

 different characteristics. 



1. The inner zone is highly developed; it is much thicker than the 

 two other layers taken together. It is of a lamellar structure, as on 

 cross-section it presents numerous concentric striae. This inner zone 

 is slightly affected by acid stains. 



2. The median zone is much less thick than the inner, but much 

 thicker than the external zone. This zone or layer forms the thick- 

 enings which project exteriorly in the form of longitudinal ridges on 

 the surface of the integument. It follows that it is much thinner 

 at the level of the intercostal furrows than at the level of the ridges. 

 It appears not to be lamellar in structure, but rather to present at 

 the level of the ridges vertical striae. Finally, its most remarkable 

 property is its great affinity for basic stains. 



3. The external zone is extremely thin; unless high magnifications 

 are employed to observe it, it easily escapes detection. It covers the 

 median zone in the intercostal furrows as well as on the ridges, and 

 is not thicker at their level. It is not affected by either basic or acid 

 stains. This layer forms the hairs. Neither the median nor the 

 internal zone takes part in the formation of hairs. 



It follows from the difference in affinity with regard to stains, 

 shown by the two first layers, and the inability to stain the outer 

 layers, that the three chitinous layers are easily distinguished by a 

 combined staining method. By means of magenta red and picric 

 indigo carmine, for instance, the inner zone becomes greenish and the 

 median zone dark red, while the external layer remains colorless. 

 Similarly, by a combination of hematoxylin and light greeti, the inner 

 layer stains green and the median layer deep black, the external 

 layer always remaining colorless. 



The descriptions of the chitinous integuments of insects do not 

 completely harmonize with those given by Lecaillon of Tabanus 

 quatuornotatus. Lecaillon assumes two reasons for this fact: first, 



