cover photo from Shoot for Science by Deepak Kakara , Dinesh Yadav Sukanya Olkar, and Parijat Si
By Sebastian Sturm – Here in the NICE lab, we work on Rhagoletis pomonella – the apple maggot. The apple maggot is the larva of a fly which is specialized to a particular food source. Surprise: It’s the hawthorn berry. To us, hawthorn berries and apples appear very different. Not so to the apple maggot fly – it likes to lay its eggs in both. However, they first experienced apples once European settlers introduced the fruits to North America. Before that they were only hawthorn berry maggots. Rhagoletis is a member of the family Tephritidae – fruit flies – but unlike Drosophila which is another fly family. The Tephritids are a family with more than 4000 members that cause enormous damage in agriculture.
Insects are the most diverse group of animals – not only regarding their number of species but also regarding their various life styles. Insects can be found in the air, water and land – ranging from desserts to glaciers. Just as diverse as their habitats are their food sources: liquids and solids of plant or animal origin. Insect species are often highly specialized to a particular source. Hence it is not surprising that their digestive system exhibits one or another interesting structures and adaptations – like the crop.
The first time I was dissecting the nervous system of an adult fly of Rhagoletis pomonella I was surprised by the staggering size of its crop – which literally filled the entire body of the fly. Because it was extensively contracting it provided an extraordinary view. I called my lab mates to have a look. Disgust mixed with curiosity: “What the heck is that?”
The crop of Rhagoletis pomonella is a large, two-lobed, thin-walled and transparent balloon. It is located between the oesophagus (the pipe for the initial food transport) and the proventriculus, a muscular ring which functions as a valve between foregut and midgut. In flies like Rhagoletis and Drosophila, the crop is connected to the oesophagus via a junction. The fly ingests the liquid food along with digestive enzymes secreted from glands. This solution is stored in the crop, which is contractile and thoroughly mixes its content and passes it to the proventriculus. The proventriculus regulates the further transport in the midgut. Therefore, the general function of crop and proventriculus correspond to our stomach.
Some insect groups utilize their crop as a defensive mechanism. They can bring up previously swallowed food to discourage and disgust predators. Locusts and cockroaches for instance throw up a very dark unappetizing goop when they feel threatened and captured. I feel that cockroaches themselves are already unsavoury enough, but I guess they’d rather be safe than sorry.