It will depend on the aim of your research.

what is your objective?

Thank you Andreas, but in the excel file i have three defined columns ( repetition, length, width ) , i have tried with discrimination but it doesn't work may be because i couldn't arrange well the data in the excel file?

Dear Silva, My objective is to find out from 50 random larvae within the population, the number of possible instars that the scolytus have. in other word to classify them into classes of size using two combined variables (length and width of larvae mandibles)

Hi Asma Zeiri.

I think you may first increment the number of individuals in oorder to run a more accurate analysis. I assume that you are using individuals who were reared in the same host plant and int he same conditions; it they do not rear in the same conditions you have to make a first analysis to determine if there are differences in your measurements due to rearing conditions.

In any case, the best way to make an analysis is grouping the data by frequencies and them analyse them. I have a paper published about a similar issue you are addressing, and you can download it from my profile or clinking in this link:

https://www.researchgate.net/publication/232663556_Head_Capsule_Width_and_Instar_Determination_for_Larvae_of_Streblote_panda_%28Lepidoptera_Lasiocampidae%29?ev=prf_pub

If you have any question please do not heasitate to ask,

David

Article Head Capsule Width and Instar Determination for Larvae of St...

Hi David, thank you so much for the answer and for the publication, yes, in fact, i have same rearing conditions, same host, and same population, more measurement are needed , i am with you , but for the moment 50 larvae is the number that i had in the same "condition of rearing" . I will read your paper first , then i will ask you about it, sure it is very interesting work. Did you run some other tests about adults measurements and the effect of the population competition on their size ?

Best regards

Asma

Hi Asma. I did measurements on adults, but in my case there were lepidoptera so I measured the length of the forewings and also dissected the adutls females in order to obtain fecundity parameters. In my lepidoptera the adults do not feed, so all the resources the adults have depend on the larvae. Also this larvae is able to increase their development time and also their instar number, in order to obtain a minimum adult size and weight.

David

Interesting , thank you for sharing

Try Bhattacharya method or its equivalent by maximum likelyhood. See for instance

http://www.unuftp.is/static/files/short_courses/Stock_assessment_fiji06/M42_Age%20and%20Growth.pdf, slides 24 and seq.

Thank you