When Dave and I were working on the spa bath deck, I happened to notice a stick bug on the roof post of the gazebo. Then I noticed that there was a smaller stick bug on her back!
When both sexes of stick insect exist, the female is always larger. The smaller males are always more difficult to identify than females, and in some cases, impossible to identify unless seen with a corresponding female.
When reproducing sexually, the female will often carry the male on her back for several hours to a few weeks (!), pausing their buggy sex only to lay eggs (they can eat independently during this time--just picture a buffet of leaves instead of strawberries and chocolates).
Well, in some stick insect species, males are not known to exist, or to occur very infrequently!
An entire genus (Acanthoxyla) consists of only female organisms, with one exception (A. senta).
So, how does the species perpetuate itself if only females exist? The females reproduce via parthogenesis, a process nicknamed "virgin birth." In the case of stick insects, unfertilized eggs become female stick insects, and the cycle can continue (fertilized eggs become male stick insects). Plenty of other animals can reproduce via parthenogenesis, most famously the Komodo dragons.
Salmon, J.T. "Stick Insects." Tuatara : Journal of the Biological Society. 5.3 (1995): 77-81.