The silk and silk production system of the funnel‐web mygalomorph spider Euagrus (Araneae, Dipluridae)

The silk production systems of several specimens representing at least three species of the mygalomorph genus Euagrus were examined by dissection and by a variety of absorption and fluorescent histochemical techniques. The intrageneric variation is primarily restricted to gland number, with larger numbers of glands in the larger-bodied species. The silk glands are arranged in four groups each serving one of four spinnerets. Spigot morphology is uniform, consisting of a long, slender shaft of slightly variable length emerging from an enlarged sac-shaped base. Only one gland type is identified histochemically, although a considerable range in size and shape is observed. Each gland has three secretory zones, a distal one that produces a largely hydrophobic protein core and medial and proximal zones that produce distinct charged protein coatings. Amino acid analysis of the silk indicates a composition very similar to the frame threads and draglines of araneomorph spiders. The high percentage of short side-chain amino acids implies a polymer structure able to support the suspended entrapment web found in Euagrus and its relatives (but which is rare in most other nonaraneomorph families). Dry silk examined under high magnification appears smooth and homogenous. The fact that in the web larger threads appear to fibrillate suggests that Euagrus silk is polyfilamentous.