Rather than relying on images (often not reliable when trying to differentiate between very similar species), you might have a look at the publication “The Amphipod Genus Arcitalitrus (Crustacea: Amphipoda: Talitridae) of New South Wales Forests, With Descriptions of Six New Species” [http://amonline.net.au/pdf/publications/1459.pdf] where the following description occurs: “Arcitalitrus appears to have two groups. The “Sylvaticus”species group is defined by gill 6 that has a convoluted anterior margin and a subapically incised posterior margin and the rami of pleopod 3 that are subequal in length to the peduncle. This group contains Ar. bassianus, Ar. belbucca, Ar. nana, Ar. orara, and Ar. sylvaticus. The “Dorrieni” species group contains Ar. dorrieni and Ar. moonpar and is defined by epimeron 2 that is longer than epimeron 3, a subquadrate posteroventral corner on epimeron 3 and an entire telson. The long epimeron 2 and
the subquadrate corner of epimeron 3 are homoplastic characters that also occur in Ar. bassianus.
If it would help, there is a diagram of generalized amphipod morphology at http://tinyurl.com/d3cuuc
