Occurrence of Chitin in the Epicuticle of an Arachnid Palamneus swammerdami

Abstract

IN a previous communication¹, it has been pointed out that the epicuticle of Palamneus swammerdami conforms to the basic pattern suggested for insects in consisting of a non-chitinous lipoprotein layer overlaid by a layer of wax. However, more extended observations carried out since then indicate the presence of chitin in this layer, which is masked in some way so that it is not readily revealed by the usual histochemical tests. Blower² reported that in myriapods a layer of the cuticle, which he considered the outermost, resembles the epicuticle of insects in histochemical and staining reactions. Only the presence of chitin in it prevented him from homologizing it with the epicuticle. However, he ignored the presence of a layer external to the one he described as the outermost, for he doubted its histological validity, considering it as a diffraction effect. In a recent study of Palamneus ^1,2, it has been shown that the layer referred to by Blower corresponds to the exocuticle and is distinct from the epicuticle, which lies external to this layer. Cloudsley-Thompson⁴ also described in arachnids and myriapods a similar colourless epicuticle external to the tanned exocuticle. The present communication gives the evidence for the presence of chitin in the epicuticle.