Moult-related changes in ampullate silk gland morphology and usage in the araneid spider Araneus cavaticus

Abstract

Major ampullate (MaA) and minor ampullate (MiA) silk glands of juvenile Araneus cavaticus (third to penultimate instars) were examined by dissection at various times relative to ecdysis. Several days before ecdysis the larger pairs of MaA and MiA glands become non-functional and remain so until ecdysis. Nevertheless, proecdysial spiders are able to draw ampullate fibres due to the presence of smaller pairs of MaA and MiA glands which are functional at this time. Indeed, it appears that these smaller ampullate glands are intended for use only during proecdysis. Thus, larger MaA and MiA glands and smaller MaA and MiA glands are typically not used concurrently (a brief transitional period is an exception). The smaller ampullate glands functioning in one juvenile stadium regress in the following stadium and become (what have previously been referred to as) accessory MaA and MiA glands. These nonfunctional accessory ampullate glands do not re-develop into functional smaller ampullate glands until the following stadium. Thus, a given pair of smaller MaA or MiA glands is only functional in every other juvenile stadium. However, because there are two sets of smaller/accessory MaA and MiA glands which function alternately, the spider is able to produce ampullate fibres during the proecdysial portion of each stadium. A new terminology for the larger, smaller and accessory ampullate glands is proposed which emphasizes the kinship between the two sets of smaller/accessory ampullate glands.