title: Internal receptors in insect appendages project directly into a special brain neuropile.
abstract: BACKGROUND
  The great majority of afferent neurons of insect legs project into their segmental ganglion. Intersegmental projections are rare and are only formed by sense organs associated with the basal joints of the legs. Such intersegmental projections never ascend as far as the brain and they form extensive ramifications within thoracic ganglia. A few afferents of chordotonal organs of the subcoxal joints ascend as far as the suboesophageal ganglion.
  
  
  RESULTS
  We describe novel afferent neurons in distal segments of locust legs that project directly into the brain without forming ramifications in other ganglia. In the brain, the fibres terminate with characteristic terminals in a small neuropile previously named the superficial ventral inferior protocerebrum. The somata of these neurons are located in the tibiae and tarsi of all legs and they are located within branches of peripheral nerves, or closely associated with such branches. They are not associated with any accessory structures such as tendons or connective tissue strands as typical for insect internal mechanoreceptors such as chordotonal organs or stretch receptors. Morphologically they show great similarity to certain insect infrared receptors.We could not observe projections into the superficial ventral inferior protocerebrum after staining mandibular or labial nerves, but we confirm previous studies that showed projections into the same brain neuropile after staining maxillary and antennal nerves, indicating that most likely similar neurons are present in these appendages also.
  
  
  CONCLUSIONS
  Because of their location deep within the lumen of appendages the function of these neurons as infrared receptors is unlikely. Their projection pattern and other morphological features indicate that the neurons convey information about an internal physiological parameter directly into a special brain neuropile. We discuss their possible function as thermoreceptors.
authors.affiliation: RWTH Aachen, Institut für Biologie II (Zoologie), Abteilung für Entwicklungsbiologie und Morphologie der Tiere, Helmertweg 3, Aachen, D-52074, Germany. braeunig@bio2.rwth-aachen.de.
authors.forename: Peter
authors.initials: P
authors.lastname: Bräunig
authors.forename: Katharina
authors.initials: K
authors.lastname: Krumpholz
references.citation: J Microsc. 1979 Mar;115(2):161-3
references.pmid: 85714
references.citation: J Exp Biol. 1996;199(Pt 6):1383-93
references.pmid: 9319276
references.citation: J Embryol Exp Morphol. 1979 Dec;54:47-74
references.pmid: 575157
references.citation: Cell Tissue Res. 1981;216(1):79-96
references.pmid: 7226210
references.citation: J Neurosci. 1988 Nov;8(11):4349-66
references.pmid: 2846797
references.citation: Microsc Res Tech. 1997 Dec 15;39(6):506-31
references.pmid: 9438251
references.citation: Proc R Soc Lond B Biol Sci. 1955 Jan 27;143(911):281-92
references.pmid: 14371609
references.citation: Cell Tissue Res. 1981;216(1):57-77
references.pmid: 7226209
references.citation: Dev Genes Evol. 1998 May;208(3):142-50
references.pmid: 9601987
references.citation: J Exp Biol. 1995;198(Pt 8):1673-89
references.pmid: 9319581
references.citation: Oecologia. 2004 Jan;138(2):312-9
references.pmid: 14614620
references.citation: Brain Behav Evol. 2001 Jan;57(1):1-17
references.pmid: 11359044
references.citation: J Comp Neurol. 1991 Oct 22;312(4):493-508
references.pmid: 1761738
references.citation: Cell Tissue Res. 1981;216(1):97-111
references.pmid: 7226211
references.citation: J Comp Neurol. 2000 Sep 11;425(1):82-96
references.pmid: 10940944
references.citation: J Exp Biol. 1977 Feb;66(1):221-41
references.pmid: 870600
references.citation: Philos Trans R Soc Lond B Biol Sci. 1987 Jan 27;315(1168):1-36
references.pmid: 2881311
references.citation: Arthropod Struct Dev. 2013 Mar;42(2):135-42
references.pmid: 23178564
references.citation: J Exp Biol. 1974 Jun;60(3):673-705
references.pmid: 4367892
references.citation: J Comp Neurol. 2010 Oct 1;518(19):3907-30
references.pmid: 20737592
references.citation: J Neurobiol. 1982 Sep;13(5):413-33
references.pmid: 7130981
references.citation: Arthropod Struct Dev. 2001 Nov;30(2):99-111
references.pmid: 18088948
references.citation: Arthropod Struct Dev. 2000;29(4):295-305
references.pmid: 18088935
references.citation: Curr Biol. 1994 Aug 1;4(8):671-5
references.pmid: 7953552