Egg sac silk of Theridiosoma gemmosum (Araneae: Theridiosomatidae)

Abstract

Cocoons of Theridiosoma gemmosum consist of two main parts, the egg sac case and the stalk. The inner space of the egg sac case is filled with nonsticky flocculent silk. Measuring 600-800 nm in diameter, the flocculent threads are never made up of bundles of longitudinally oriented nanofibrils. The egg case wall consists of a lower layer of highly ordered threads and an upper layer of cover silk. The lower, permanently white layer consists of threads in a mesh-like arrangement, the thicker threads being 4-6 microm and the thinner threads being 2-3 microm in diameter. Each thread is a bundle of parallel nanofibrils, with a diameter between 150 and 300 nm. The silk secretions of these fibers, emitted from spigots, are processed by legs. The upper layer of the egg case is applied to the threads of the lower layer by direct rubbing against its surface, i.e. without the use of legs. In the lower and middle part of the egg case, the accumulated secretion forms a virtually compact encrustation, whereas in the upper, conically shaped, part of the egg case where it becomes the stalk, this secretion becomes substantially scarcer. The stalk is a continuation of the egg case, its proximal part made of fibers similar to those forming the inner layer of the egg case wall. The distal part of the stalk continues towards the suspension area either as a compact bundle of parallel fibers, or the stalk forks into two bundles of roughly the same thickness, which continue towards the suspension area separately. On the surface of objects onto which cocoons are attached, the secretion of the piriform glands acts as an adhesive sheet.