Araneus angulatus

Palmgren, P., 1980, Some comments on the anatomy of spiders, Ann. Zool. Fennici 17, pp. 161-173 : 164-166

Araneus angulatus

All araneid species previously dissected by me are fairly small, the biggest being Araneus cornutus , whereas Nephila and Cyrtophora include larger species. For the sake of comparison, I dissected a female of Araneus angulatus Clerck (Figs. 14-18), which is of about the same size. The cheliceral m.lateralis anterior (la) is as clearly doubled as in Cyrtophora . Of the m.tergo-pedipalpalis muscles, the medius [pm) is two-fanned, the others are simple. A m.tergo-pedipalpalis externus (pe) is present, which is rare in non-amaurobid spiders. This finding corroborates the view previously expressed by me (1978 a), that bigger species need a more diversified set of muscles. In A.angulatus the dorsal caecal pouch proved to be very long and narrow, extending well into the basal portion of the chelicers, leaving ample space for the poison gland parallel to the caecal pouch.

The musculus lateralis (Ml) is noticeably weaker in Nephila than in Araneus , the fibres being shorter. The cause of this is probably the completely sclerotized pleurae (connecting carapace and sternum) of Nephila , which apparently allow only a very small amount of depression of the margin of the carapace. In Araneus angulatus (and in A.cornutus ) the pleurae contain discrete, triangular sclerites. The musculus lateralis has generally been credited with the generation of a raised body fluid pressure, causing extension of the leg joints lacking extensor muscles. It remains to be shown how Nephila generates sufficient power to straighten the very long legs. The coxae must also be much less mobile in Nephila than in Araneus . This is reflected by the absence of musculi tergo-coxales posteriores profundi (c 4, "posterior rotators"), whereas A.angulatus has such muscles in legs I-II, A.cornutus in I-III. They arise from the small sclerites in the pleurae. These muscles have been found chiefly in the fairly large-sized spiders belonging to the amaurobides complex, sensu Lehtinen. Cf. the m.tergo-pedipalpalis externus of A.angutalusl The systematic value of these muscles should clearly not be overrated, as the crucial factor determining their presence is simply a large body size (Palmgren 1978a:19).

It is perhaps worth mentioning that Azilia , Cyrlophora and Nephila all have a pair of strong muscles running fiom the lorum backwards to the hind margin of the carapace ( Itß). The same muscles were found in Araneus cornutus (but not in A.angulatusl ), in Drassodes , Callilepis (Gnaphosidae) and in Clubiona . The interpretation seems difficult: is this muscle a posterior portion of m.lorotergalis (cf. Whitehead & Rempel) or the hindmost portion of the lateralis muscle?

The above-mentioned traces of resemblance to tetragnathid spiders do not, in my opinion, alter the isolated position of that group. Cf., however, the arguments of Levi in a most recent publication (1980)!

Figs. 19-20. Comaroma simoni . - 19: ventral surface of opisthosoma, petiolus cut. - 20: organs immediately adjacent to the integument.