crayfish?

The two first looks like Chironomidae eggs .

Dear Jean Francois,

I think they might be wing membranes with microtrichia. It's a possibility.

There is a similar image here, if you zoom on it.

http://www.dipteristsforum.org.uk/img/gallery/13269_1335695084.jpg

Lucian

I like the idea that they are wing membranes. My concern is that there are no veins. However, I have no idea what the scale is in Raquel's pictures. It could be the cuticle of an insect larva. After a few hours/days all the digestible parts are gone leaving you with nearly transparent cuticle and some spines.

Hi

To me it looks like egg mass. Mabye it has eaten someones eggs. I've also heard of at least one species of frogs laying eggs before swallowing their own eggs. The eggs are kept in the stomach for some time before they give birth through the mouth. This might be rare, though. I'm not a frog expert.

Kind regards, Halldis Ringvold

The microtrichia of wing membranes was a good theory but it was discarded because of the size of the spines, absence of veins and because they were always associated with gelatinous masses.

After Jean François answered, I did some research and they do really look like chironomid eggs (http://www.biolib.cz/en/image/id88447/).  

Chironomid larvae is one of the most abundant items in the diet and pieces of plants were present in almost all the stomachs that contained this structures, where they could have been laid.

Thanks for all the help! 

I work with macroinvertebrates (fresh and estuarine) and I can't identify this structure but it seems familiar. It looks like chironomid eggs but I think that it is another structure because eggs are always inmersed in a mucilage. 

Tadpole teeth? 

or Maybe polychaete pharynx?  if you are near marine environment (coastal marsh...)

Sorry. unknown to those structures.It is not my issue.

Dear Raquel, just came across your question. The structure immediately made me think to Xenopus shed skin. Frogs usually ingest their skin while shedding. Since we have some Xenopus in the lab I checked for some shed skin in the aquaria. I found a very small piece that I put under the microscope and what I saw is extremely similar to your photos. Hope this helps. Cheers, Philippe.

I agree with Mr. Kok. The epidermis of some very small lizards have this same beaded appearance. Bits of materia similar to this material are sometimes encountered in the droppings of predatory lizards in the Mojave Desert in California.

Alfonso, i've already looked at the tadpole teeth of the only other amphibian species (Pelophylax perezi) that occurs at the stream where these frogs were captured, and they have a different shape. 

Philippe, Xenopus skin was my first hypothesis, since they are known to eat their skin, but they have black star-shaped chromatophores and, although it's not easily perceived in the photos, these are brown conic structures. I've also found a few X. laevis tadpoles in the stomachs and their skin was different.

Thanks again for your interest and suggestions! 

Hi Raquel, I perfectly see these conic structures, in my opinion they correspond to the tiny spicules present in Xenopus skin. I can see the exact same on the shed skin I examine under the microscope (unfortunately no camera connected otherwise I am sure I would convince you!). I just asked a student of mine to compare with your photos and she agrees this is strictly identical. I thus still believe that what you report is shed skin (upper layer of the frog skin to make it simple). One option to confirm this is to extract DNA from your samples, sequence 16S and Blast the sequence. Cheers, Philippe.

Here is a photo of a piece of Xenopus shed skin (taken from an aquarium containing Xenopus). Quality of photo is bad because I took it with an iPhone through the microscope lens. Anyway, compare it with your photos. You can see the dark spicules I mentioned in my previous message. Note that in your case shed skin is probably already slightly altered by gastric acid. Still doubting ;-) ?

Not anymore!

The spicules in your photo look exactly with what I saw in the stomachs even considering, as you said, that the skin was slightly altered. I assumed that the skin would look like the tadpoles skin in the stereomicroscope... Lesson learned!

Thank you for the time you spent searching and explaining this to me, Philippe. I will not forget your help in my master thesis acknowledgements. 

I agree with Philippe on that. Likely Xenopus skin. This is not unkommon for amphibians to feed on their own skin or even fed by their mothers skin ;-). But see in the review by

Weldon, P. J., Demeter, B. J. & Rosscoe, R. A survey of shed skin-eating

(dermatophagy) in amphibians and reptiles. J. Herpetol. 27, 219–-228 (1993). 

Bests

Alex