Who is this little critter, and why is he so happy?
This is Hymenolepis diminuta, the rat tapeworm. It doesn't look much like a tapeworm just yet, because it's in an intermediate stage of development, called a cysticercoid. This larval stage of tapeworm lives in beetles.
We raise flour beetles in our lab to serve as a host for these larval stages of tapeworm. The beetles find the eggs pretty tasty. Once they've been eaten, the eggs hatch in the beetle gut, and the parasites use hooks to migrate into the beetle's body cavity. There, they undergo some changes (including the development of a protective cyst), and become a mature larval tapeworm. Our current lab record is 35 of these cysticercoids in a single beetle.
In a non-lab setting, some of these infected beetles would be eaten by a rat. In fact, the parasite modifies the behaviour of the beetle to help increase the odds of that happening. While the infected beetles don't look any different than the uninfected ones, they act very differently. The infected beetles move more slowly, are less fearful of the light, don't respond to sex pheremones. These changes in behaviour are only present when the larval stage of parasite is fully developed (it takes about 2-3 weeks), suggesting that these behavioural changes are mediated by the parasite.
In our lab, we squish up the infected beetles in a petri dish with water, and hunt down the little cysticercoids that pop out. We carefully count them out, and place 5-10 of them in a little tube, filled with salt water. The larval parasites, along with the salt water, are fed to mice or rats (depending on the experiments being done). The parasites are swallowed, and when the pH levels are nice and high, they hatch out of their cysts. This ensures they hatch in the small intestine -which has a high pH thanks to the digestive enzyme trypsin-where they want to be, rather than the stomach, which would mean certain death. They stretch, find a nice intestinal villi to snuggle up to, and bathe in a seemingly never ending supply delicious nutrients.
Maybe it can sense that's where it's headed. Maybe that's why it looks so happy.