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ptilinum, ptilinal suture

Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
No Taxon (Glossary)
No Taxon (P)
No Taxon ptilinum, ptilinal suture
Explanation of Names
From Greek ptilon feather.
Identification
ptilinum noun, plural ptilina - an eversible, membranous sac found underneath the frons in schizophorous dipterans (i.e. Calyptratae and Acalyptratae) and used for rupturing the puparium(1).

ptilinal suture - a groove above the antennae, roughly crescent-shaped and oriented downward (see "7" in Fig. 1 here), which marks (and is a remnant of) the curve along which the ptilinum was permanently withdraw after its outward expansion from the top of the head during eclosion.

[Note: These terms are sometimes misspelled as "ptilinium" and "ptilinial suture" (as was the case earlier in this entry...corrected 5 June 2010).]
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More Detailed Explanation: The ptilinum is an inflatable, membranous sac present under the frons in final-stage pupae of schizophoran flies.
Initially empty, the ptilinum is rhythmically pumped full of haemolymph and thrust through the ptilinal suture as an aid for breaking out of the pupal case during emergence of the adult from the puparium.
During this process the front of the head capsule splits along the ptilinal suture to allow for the outward expansion of the ptilinum. After the head capsule splits open, the area of the face below and within the the curve of the ptilinal suture folds downward along its lower edge (that lower edge acting as a "hinge").
The ptilinum is also used for tunneling through to the surface from within any substrate in which the puparium may have been sheltered (e.g. a plant host or insect host, soil, etc.) (2).
Soon after eclosion is concluded the ptilinum is retracted back into the head, which closes up (again like a hinge) leaving the ptilinal suture as the only visible (scar-like) vestige of the process.
Remarks
There are a number of very interesting posts on BugGuide showing flies with their ptilinum in various states of inflation... either while eclosing from their puparium, or as recently emerged tenerals:
         
Internet References
Wikipedia--Ptilinum
Works Cited
1.Principles of Insect Morphology
R. E. Snodgrass. 1935. McGraw-Hill Book Company.
2.A Dictionary of Entomology
George Gordh, David H. Headrick. 2003. CABI Publishing.