Use mathematical morphology, see top-hat operation (not direct but 1-TH)

Some ideas for you:

- Threshold with Otsu and keep the object with the largest area;

- Threshold + Region growing of the object with larger area.

if you have a few more manually segmented images like the one you show above, one solution is using U-net model for segmentation.

example of implementation:

https://github.com/jocicmarko/ultrasound-nerve-segmentation

However, consider to use other conventional image processing approaches if you are not dealing with very sophisticated images.

Draw a histogram of the pixels. Then locate the bin value that mainly covers the insect defining area. Let this bin in the histogram is BinInc. In the given image, the pixels at the tail and along the border of the insect are close to the BinInc value. Hence, these value containing bins will be the neighbors of BinInc in the histogram. You can form binary image by modifying pixel values of (BinInc to BinInc-R) to 255 and the rest to 0. You can analyse for R value of 1 to 50 to find a suitable R. For better result, apply partitioning the image into blocks and then apply the stated method. Thereafter, apply morphological closing and opening parameters to reduce noises.

By subtraction two sequences image of moving object(if you have video stream) , then extract the object.

For your case where foreground is significantly different from background you can also use active contour segmentation in Matlab.

Thanks everyone for your help. I tried some of the methods you suggested which are top-hat transform and active image segmentation. Still the methods don't mask entire animal including its tail hair. I tried below steps and found it the best for now. ImageJ was used.

1. Convert images into 8bit grayscale.

2. Apply white top hat with rad=4.

3. Subtract the result of step 2 from the result of step 1.

4. Apply the threshold with relevant intensity. Threshold value is found 18 in my case.

I attached the resultant image.

I think if I can somehow increase the intensity level of the tail hair region before step 4, then that region should also give desired result.

Let me know guys if you can get better result.

Canny edge detection

Threshold otsu method

Morophlogical operation

Can help u to segment ur object

Edge detection method may be useful as seems that edges will only belings to the object, subsequently morphological operations to refine the results. Thresholding should work also as the object seems to be very sharf and bright as compares to baxkground.