The Filippi’s Glands of Giant Silk Moths: To Be or Not to Be?

Abstract

The Filippi’s glands (FGs), formerly “Lyonet’s glands”, are paired accessory organs associated with the silk glands. They are unique to Lepidoptera caterpillars and their exact role is not clear. The FGs are thought to be involved in the construction of a silk cocoon in bombycoid moths. FGs can differ in size and shape, therefore, in this study we attempt to find a correlation between FG morphology and phylogenetic position within the Bombycoidea. We use light and electron microscopy to examine the presence and morphology of FGs in a range of wild (giant) silk moths and several related species. Our results confirm that the majority of studied silk moth species have complex type of FGs that continuously increase in size during larval development. We identified several species of giant silk moths and two hawk moth species that completely lack FGs throughout their larval development. Finally, in several hawk moth species in which FGs are well developed during the first larval stage, these glands do not grow and remain small during later larval growth. Our results suggest that FGs are not critical for spinning and that loss of FGs occurred several times during the evolution of saturniids and sphingids. Comparison of FGs in different moths is an important first step in the elucidation of their physiological significance.

The Role of Filippi’s Glands in the Silk Moths Cocoon Construction

“…The most ancestral Saturniid species examined in this study, A. tau, has no FGs and forms a sparse silk cocoon (Supplementary Materials Figure S1C). [3,16] and cocoon structure of A. mylitta, A. pernyi, A. yamamai, S. pavonia, S. pyri, and C. promethea was previously studied by Chen et al [19]. The types of cocoons of the other species in this study are indicated in Figures 1 and 2 and Figure S1 in the Supplementary Materials.…”

“…To further support this conclusion, we also compared all published results demonstrating the presence or absence of FGs in giant silkworms [3,16] with their respective cocoon architecture [18,19,21]. For the following species, Saturnia pavoniella, Antherina suraka, and Aglia tau, where the complete absence of FGs has been demonstrated [16] and whose cocoon structure has not yet been described in the literature, we used SEM to show that they form compact cocoons (Supplementary Materials Figure S1). The correlation between the presence of FGs and the type of cocoon architecture is shown in Figure 3.…”

“…In our previous study [16], we identified six species of giant silkmoth (Saturniidae) that lack FGs throughout their larval development. Here we were interested whether the presence of FGs had effect on the type of cocoon constructed.…”

“…Considering that the complete removal of FGs in the larvae of the silkworm B. mori had no obvious effects on silk spinning behavior and silk quality [14,15], it is reasonable to assume that the role of FGs in cocoon spinning is probably not crucial, but rather complementary. Recent studies on some giant silkworm caterpillars (Saturniidae, superfamily Bombycoidea) have shown that FGs are completely absent throughout larval development in some of these species, although they spin remarkable silk cocoons [16]. It is known that giant silk moths can spin cocoons that consist of multiple layers of silk.…”

“…In this study we investigate a correlation between the presence or absence of FGs, and the type of cocoons constructed. We have previously shown that Saturniid species with well-developed FGs always spin compact cocoons, while species without FGs build loose cocoons [16]. To confirm the function of FGs in cocoon compactness, we surgically removed FGs in the last larval instar of the silkworm B. mori.…”