1877.] The. Study of Zodlogy in Germany. 399 
attached along their whole base, but are partly drawn back, so 
that there is a space (sp) between the middle of the base and 
the muscular walls of the crop. The teeth form six regular, 
longitudinal rows, numbering each about twenty teeth. Their 
form varies according to the genera, and probably also accord- 
ing to the species. The walls of the crop are built up mainly of 
circular muscular fibres (muc) which by their contraction drive 
the teeth towards the centre and so grind up the food of the 
cricket, thus performing a function which we are wont to think 
of as properly belonging to the mouth. The study of the devel- 
opment of the teeth enabled Herr Wilde to ascertain that they 
are formed by underlying cells through a series of transforma- 
tions of the cuticula, which appears at first as a simple membrane 
and then develops the secondary projections, which give the 
teeth their ultimate form. All these interesting discoveries 
could hardly have been made except by means of sections. 
The author has himself applied section-making to the study of 
the trachea» of insects.1 It was found that the current descrip- 
tions in works on comparative anatomy and entomology were 
incorrect in several important particulars. The outside of the 
trachea is covered by a layer of flat polygonal cells, or, as it is 
called, a pavement epithelium. Thus in a longitudinal section 
of the main tracheal stem of the common water-beetle, Hydroph- 
ilus (Figure 73), the thin cells (ep) may be easily rec- 
ognized by their nuclei. The epithelium secretes the f i 
enormously thick and complicated cuticula (ew) which Y/U 
makes up the rest of the tracheal wall. The well- {|Q 
known spiral threads or filaments ff are part of this | WA 
cuticula, and not distinct structures as was generally 4)|) 
Supposed. These threads run around the tubes and 
serve as elastic supports to keep the thin walls dis- 
tended ; they are more or less spiral, but instead of || "7s 
there being but one single thread, as is usually stated, \ cd 
there are four or five which end, after making a few (ro. 73,) LON- 
turns around the tracheæ, new ones arising to replace pean a 
em. As the fibres run transversely, of course their THE fT 
s CHEA 
cut ends only are seen in a longitudinal section io Sa 
Figure 73. But these ends show that the filaments con- 1US PICEUS. 
‘ sist of a lighter outside, and a darker inside portion, which latter 
round. The rest of the cuticula (cu) is divided into two layers, 
1 Minot, Recherches histologiques sur les Trachées de sh drophilus piceus. Arch. 
de Physiol. normale et pathologique, sér. ii., tom. iii., page 1. 
